10 Essential Rules of Herbal Safety:
A Comprehensive Guide to Using Medicinal Herbs Safely

Introduction to Herbal Safety

Medicinal plants have been my primary form of health care for the last 35 years, and I feel privileged to have shared the joy of herbal healing with thousands of students over these decades. And as an aging herbal hippie, I’ve seen some things. Herbs-gone-wrong scenarios. Friends eating the wrong mushroom or plant, resulting in an all-night prayer vigil to the porcelain God. Thankfully, my comrades did not meet their maker.

But others have not been so lucky. A close friend’s classmate sadly died from mistaking foxglove for comfrey leaves and adding them to her smoothie.

My former roommate suffered chemical burns that unfortunately lasted for years after “freshening up” her armpits with a lemon leaf. It turns out the essential oils in citrus leaves contain compounds that cause rashes when exposed to the sun (in scientific speak: oxypeucedanin and bergapten are photosensitizing compounds found in the essential oils contained in lemon leaf).

I’m not telling these stories to scare you away from foraging and medicinal plants! Rather, I’m sharing them to explain why herbal safety is essential, why I created this guide, and why I’ve included herbal safety lessons in all my online programs. With these core tenets of herbal safety under our belts, we are more equipped to enjoy and share botanical medicine.

When we first learn about herbalism, it’s all too easy to assume that plants pose little risk because they are natural. However, it’s crucial to pay attention to herbal safety during pregnancy, nursing, and with children and elders. We also want to be mindful of herb-drug interactions. And it goes without saying: proper plant identification is central to staying safe as a forager.

Knowing the correct dosage and recommended duration for taking an herb is another core aspect of herbal safety. I’ve also witnessed the harm resulting from not seeking proper medical attention due to dogmatic beliefs around natural therapies. Some well-meaning but misguided people overestimate the value of herbs and underestimate the importance of diagnostic care and conventional interventions.

Finally, herbs and herbal remedies can be contaminated or adulterated with the wrong species, either intentionally or by mistake. While herbs are generally safer than most pharmaceuticals and have fewer side effects, it is crucial to be aware of precautions, particularly concerning drug-herb interactions.

I hope that learning about herbal safety gives you greater confidence in using plant medicine. It’s easy to be scared away from learning all that we shouldn’t do with plant medicine, but I hope that you’ll feel even more enamored with this ancient form of healing—one that our ancestors relied on for countless generations. This article is limited in scope: if you want to go deeper, I recommend checking out all of our herbalist programs, which include a full module on herbal safety. You can also download our free guide, The Ten Essentials of Herbal Safety.

Several botanicals are toxic to the point of being deadly. Other herbs in this category are so potent that they can cause severe vomiting, diarrhea, and gastrointestinal distress. Some of these herbs may induce miscarriage due to their toxic constituents, while others can damage organs over time.

Many of these herbs were historically used for severe health conditions before safer pharmaceutical medications—with more precise and reproducible dosages—were developed. Very few contemporary herbalists work with these high-risk botanicals because of their potential for serious harm.

Some medicinal plants are toxic or potentially fatal and should only be used by highly trained practitioners—or avoided altogether. Here are some herbs I don’t work with and recommend you avoid as well (unless they are part of your healing traditions and you are thoroughly trained in their precautions and dosages). Please note that this is not a comprehensive list:

• Belladonna (Atropa belladonna)
• Birthwort (Aristolochia spp.)
• Bloodroot (Sanguinaria canadensis)
• Datura (Datura spp.)
• Digitalis, or foxglove (Digitalis spp.)
• Henbane (Hyoscyamus niger)
• Liferoot (Packera or Senecio spp.)
• Lily of the valley (Convallaria majalis)
• Mayapple (Podophyllum peltatum)
• Mistletoe (Viscum spp.)
• Pennyroyal (Mentha pulegium)
• Tansy (Tanacetum vulgare)
• Wild ginger (Asarum canadense)

As with anything one ingests, dosage makes a huge difference. Many herbal side effects occur with high dosages over an extended period. This is one of the reasons I like to use formulas with multiple medicinals—the individual herbs are in smaller doses. And I also change formulas over time and take a few weeks’ break from herbal treatment every few months.

Low-Dose Botanicals

That said, some herbs are toxic even in small doses, and they may cause side effects quickly. These botanicals blur the lines between medicine and poison. Low-dose botanicals have a greater potential for serious adverse effects and toxicity.

They should only be used by highly experienced herbalists trained in their use, including safety considerations, or traditional practitioners who have learned about these herbs through training in their lineage.

Herbs that must be taken in very small doses or avoided altogether because of potential toxicity:

• Anemone (Anemone spp.)
• Arnica (herbal, not homeopathic) (Arnica spp.)
• Blue flag (Iris versicolor)
• Lobelia (Lobelia inflata)
• Poke root (Phytolacca americana)

(Note that homeopathic arnica is considered very safe, both internally and topically, because it contains minute to untraceable amounts of herbal material. Topical herbal arnica is also generally considered safe as long as it is not applied to broken or abraded skin.)

Weight-Based Herbal Dosing (Adults & Children)

You’ll need to adjust the herb dose for people who are smaller or larger, and for children. Using weight as a starting point, you can adjust the doses of teas, tinctures, syrups, etc., to calculate the appropriate dosage. The following examples are for children, but you can use the same methods for smaller adults.

Imperial Measurements: To determine the child’s dosage by weight, you can assume that the adult dosage is for a 150-pound adult. Divide the child’s weight by 150. Take that number and multiply it by the recommended adult dosage. For example, if your child weighs 50 pounds, they will need one-third the recommended dose for a 150-pound adult.

• If the adult dosage is 3 droppers full of a tincture, the child would need one-third of that dose, which is one dropper full (⅓ of 3 droppers full).
• If the adult dosage is 1 cup of tea, a 50-pound child would need one-third of one cup (⅓ of 8 ounces), which equals 2.7 ounces (it would generally be fine to round up to 3 ounces in most cases).
• A 25-pound child would need one-sixth of the adult dose and would therefore receive one-half of a dropper full (⅙ of 3 droppers full).

Metric: To determine the child’s dosage by weight, you can assume that the adult dosage is for a 68-kilogram adult. Divide the child’s weight by 68. Take that number and multiply it by the recommended adult dosage.

• For example, if your child weighs 22.7 kilograms, they will need one-third the recommended dose of an adult weighing 68 kilograms. If the adult dosage is 3 milliliters, they will need one-third of that dose, which is 1 milliliter (one-third of 3 milliliters).
• If the adult dosage of tea is 240 ml, a child who weighs 22.7 kilograms will need one-third of that dosage (⅓ of 240ml), which is 80 ml.

Most herbs are NOT safe in pregnancy

Herbal safety during pregnancy and lactation requires heightened caution due to the potential effects of botanical constituents on fetal and infant development. When considering taking any herb during pregnancy, begin by assuming that the herb is not considered safe until you’ve verified otherwise through reliable clinical herbal resources. In other words, far fewer botanical remedies are considered appropriate during pregnancy than many people assume.

To learn more about which classes of herbs are contraindicated during pregnancy and lactation, download our free guide, The Ten Essentials of Herbal Safety.

I also highly recommend Dr. Aviva Jill Romm’s books on herbs for pregnancy, the postpartum period, and breastfeeding/chestfeeding. Bonus: She also has a wonderful book on herbal medicine for children. You can find all of our favorite titles on the topic in The Best Herbals for Women, Babies, & Children, featured in our article on the Best Herbal Medicine Books for Beginning Herbalists.

All of our online herbal programs include my detailed list of herbs to avoid during pregnancy and nursing, along with explanations of why they are contraindicated.

Medicinal herbs can be contaminated with a variety of toxic substances and contaminants, including heavy metals, herbicides, pesticides, animal feces, pathogenic microorganisms, and even prescription pharmaceuticals.

Herbal products can also be mislabeled and contain plant species different from those listed on the label. This is known as adulteration. Adulteration may occur through accidental substitution or by intentionally substituting a cheaper botanical for a more expensive one. This is one of the many reasons why sourcing herbs from reputable companies and farms is so important.

Reputable herbal companies prioritize transparency, quality control, botanical accuracy, and sustainable sourcing practices.

Some imported herbal products, including certain Chinese herbal products sold in the U.S., have been found to contain unlabeled pharmaceutical drugs such as corticosteroids, sedatives, and acetaminophen.

Here are some guidelines for sourcing herbal medicines that are accurately labeled and free from contamination and adulteration.

Look for herbal companies and farms that:

• Avoid selling endangered, threatened, or at-risk wild-harvested plants
• Grow and process their own herbs using farm-to-apothecary practices whenever possible
• Reject herbal batches that fail quality, purity, or contamination standards
• Test their herbs or herbal products for heavy metals, pharmaceutical contaminants, pesticides, pathogenic microorganisms, animal feces, and other impurities
• Use organic or regenerative growing practices
• Verify botanical identity through macroscopic, microscopic, organoleptic, or laboratory testing methods
• Work directly with growers and wildcrafters to ensure traceability and quality control

We have compiled a list of herbal sellers and medicine makers we trust. You can also explore the businesses run by graduates of our herbal school in the Chestnut School’s Student Herbal Business Directory. You’ll find herbal farms, apothecaries, and herbal product companies owned by our graduates. I couldn’t be prouder of their work, and I wholeheartedly stand behind their quality.

Accurate plant identification is one of the most critical aspects of herbal safety, especially when foraging for wild medicinal plants and mushrooms.

An Irish mushroom-harvesting adage also applies beautifully to herb harvesting:

“There are old mushroom hunters and bold mushroom hunters, but there are no bold old mushroom hunters.”

If you are growing, gathering, or wildcrafting medicinal or edible plants, you must take precautions to avoid harvesting the wrong species. You’ll also want to gather from clean environments so your harvest isn’t contaminated with heavy metals, herbicides, pesticides, roadside pollution, or other toxins.

As you’re likely aware, some poisonous plants and mushrooms can be deadly, which means one misidentification could have severe—or even fatal—consequences. Careful botanical identification is essential for anyone interested in foraging for wild foods and herbal medicine.

Here are some resources to help you be a sustainable and safe forager:

• Foraging for Wild Edibles and Herbs: Sustainable and Safe Gathering Practices
• The Ten Best Books on Foraging Wild Foods and Herbs
• The Best Regional Books for Foraging and Plant Identification

With the rise of AI-generated content and unverified herbal advice online, it’s increasingly important to evaluate the credibility of herbal education sources. Unfortunately, a crop of AI-written books has sprouted up on Amazon (accounting for 82% of all herbal books!) and other online booksellers. The author profiles are often fabricated, complete with AI-written biographies and even fake images of non-existent people. In many cases, the reviews are also AI-generated.

The dangers of AI-written foraging and herbal medicine books without proper editing, botanical verification, or human review are alarming. Some contain photos of misidentified plants or mushrooms that could lead to serious harm. Online herbal articles are increasingly being generated by machine learning as well.

At the Chestnut School, our course materials and articles are written by experienced herbalists and educators—not generated by AI.

How can I spot AI-authored herbal books and unreliable herbal resources?

Check out our article on the Dangers of AI in Herbalism.

How Do I Know Which Herbalists to Trust?

Watch out for manipulative, sensationalistic, or dogmatic tactics in herbal education. I recommend avoiding websites that discuss scientific information without including sources, citations, or references.

Be cautious of authors or educators who present health information in an absolutist or overly simplistic manner, especially when discussing complex medical conditions. Nuanced, evidence-informed herbal education acknowledges the limitations of herbal medicine alongside its strengths.

It’s also wise to be wary of herbalists, influencers, or wellness personalities who rely heavily on fear-based marketing, miracle cure claims, conspiracy-driven rhetoric, or exaggerated promises.

Herb-drug interactions can alter the absorption, metabolism, and excretion of medications, sometimes leading to reduced efficacy or dangerous side effects.

You’ve likely seen warnings about grapefruit juice interacting with certain medications or recommendations to avoid drinking black tea alongside iron supplements because it can reduce iron absorption. Similarly, medicinal plants can interact with prescription and over-the-counter medications in several ways.

Some herbs affect how drugs are absorbed, metabolized, or eliminated from the body. Others create what is known as an additive effect, meaning the herb amplifies the action of a medication with similar physiological effects.

For example, if an herb has diuretic properties, it may compound the effects of pharmaceutical diuretics. In this scenario, an individual may essentially receive a “double dose” of diuretic activity and experience excessive fluid loss. Under medical supervision, this could require adjusting the pharmaceutical dosage.

Similar additive precautions may apply to herbs that are:

• Anticoagulant (blood-thinning)
• Hypoglycemic (blood sugar-lowering)
• Hypotensive (blood pressure-lowering)
• Sedative

Herbs & Surgery

A number of herbs should be avoided for approximately 2 weeks before surgery due to known or suspected interactions with anesthesia, platelet aggregation, or blood coagulation.

Notably, St. John’s wort (Hypericum perforatum) may prolong or intensify the effects of anesthesia. To err on the side of caution, many herbalists also recommend discontinuing strongly sedating herbs and nervines approximately one week before surgery due to their potential interaction with anesthetic agents.

Herbs Commonly Avoided Before Surgery Due to Effects on Blood Clotting:

• Alfalfa (Medicago sativa)
• Angelica or dong quai (Angelica spp.)
• Fenugreek (Trigonella foenum-graecum)
• Feverfew (Tanacetum parthenium)
• Garlic (Allium sativum)
• Ginger (Zingiber officinale)
• Ginkgo (Ginkgo biloba)
• Ginseng (Panax spp.)
• Horse chestnut (Aesculus hippocastanum)
• Licorice root (Glycyrrhiza glabra)
• Red clover (Trifolium pratense)

Herb-Drug Interactions Are Highly Individualized

Herb-drug interactions must be evaluated on a case-by-case basis. Factors such as dosage, duration of use, underlying health conditions, liver function, age, and the specific medications involved can all influence the likelihood and severity of an interaction.

To determine which herbs may be contraindicated with specific medications, consult reliable references on herb-drug interactions (see the Resources section at the end of this guide) or educational resources that provide detailed lists of contraindications, such as our Online Herbal Safety Course and other herbal programs.

Herbs May Alter Drug Absorption, Metabolism, and Elimination

Botanicals may increase or decrease the absorption of medications, alter enzymatic metabolism pathways, or affect how quickly a drug is cleared from the body.

These interactions can potentially:

• Decrease a medication’s effectiveness by clearing it too rapidly from the body
• Increase drug concentrations to potentially dangerous levels
• Alter the therapeutic window of a medication

Herbs Can Have Additive Effects with Medications

Herbs may also compound the physiological effects of pharmaceuticals with similar mechanisms of action. While this isn’t always a direct molecular interaction, the combined effects may still increase the risk of side effects or excessive physiological responses.

Pyrrolizidine alkaloids (PAs) are hepatotoxic (liver-damaging) compounds found in certain medicinal plants. While pyrrolizidine alkaloids are a large class of naturally occurring compounds—some relatively benign and others highly toxic—certain forms can cause severe liver injury, including hepatic veno-occlusive disease, a serious condition that resembles cirrhosis and can potentially be fatal.

One of the most concerning aspects of PA toxicity is that symptoms may not appear until significant liver damage has already occurred. The risk associated with PA-containing plants depends on multiple factors, including the concentration and type of alkaloids present, the plant part used, dosage, frequency of use, and duration of ingestion. If exposure levels are high enough, liver damage can occur surprisingly quickly—even within a matter of weeks.

Common Herbs That Contain Harmful Pyrrolizidine Alkaloids:

• Boneset (Eupatorium perfoliatum)
• Borage (Borago spp.)
• Butterbur (Petasites spp.)
• Coltsfoot (Tussilago farfara)
• Comfrey (Symphytum spp.)
• Ragwort (Senecio spp.)

Are Pyrrolizidine Alkaloids Absorbed Through the Skin?

Pyrrolizidine alkaloids are generally poorly absorbed through intact skin. Because of this, PA-containing herbs are often considered relatively safe for short-term topical use on unbroken skin.

However, these herbs should not be applied to cuts, wounds, abraded skin, or mucosal tissue, where absorption may increase significantly. Nursing individuals should also avoid nipple salves or topical preparations containing PA-containing plants because of the potential for infant exposure.

Who Is Especially Susceptible to the Harms of Pyrrolizidine Alkaloids?

Certain individuals should avoid PA-containing herbs altogether because of increased vulnerability to liver injury related to age, life stage, alcohol exposure, or compromised liver function.

Individuals Who Should Avoid Pyrrolizidine Alkaloid-Containing Herbs:

• Children
• Heavy drinkers and people recovering from alcohol use disorder
• People with liver disease, including hepatitis
• Pregnant and nursing people

Essential oils are highly concentrated botanical extracts that require careful dilution and safety awareness to avoid adverse reactions. Because they are far more concentrated than most traditional herbal preparations, essential oils require significantly greater attention to dosage, application methods, and contraindications.

Proper dilution with a carrier oil or fat is crucial, as undiluted essential oils can cause chemical burns, skin irritation, allergic reactions, and sensitization over time. Extra caution is especially important during pregnancy and when using essential oils around babies and children, whose skin, nervous systems, and respiratory systems are more sensitive.

Certain essential oils can be particularly dangerous—and in rare cases even fatal—to infants and young children. Pregnant individuals should also exercise heightened caution with essential oils due to the potential effects of concentrated volatile compounds on fetal development. Additionally, some essential oils may become toxic when inhaled excessively or used for longer than recommended therapeutic durations.

It is important to understand that the safety of essential oils cannot be directly inferred from the herbal safety literature. Even if an herb is generally considered safe in tea, tincture, or food form, its distilled essential oil may carry numerous additional precautions, contraindications, and toxicity concerns.

The Sunflower Family Isn’t Always Sunny

Although relatively uncommon, plants in the aster, or sunflower, family (Asteraceae) can cause allergic reactions in sensitive individuals when used topically or taken internally. These reactions are often linked to compounds called sesquiterpene lactones. If you’ve reacted to one member of the aster family, you may be more likely to react to other plants within the family as well.

Medicinal plants in the aster family to be aware of include:

• Arnica (Arnica spp.)
• Chamomile (Matricaria recutita)
• Dandelion (Taraxacum officinale)
• Elecampane (Inula helenium)
• Feverfew (Tanacetum parthenium)
• Globe artichoke (Cynara cardunculus var. scolymus)
• Goldenrod (Solidago spp.)
• Purple coneflower (Echinacea spp.)
• Tansy (Tanacetum vulgare)
• Wild lettuce (Lactuca virosa)
• Yarrow (Achillea millefolium)

Watch Out for Photosensitizing Plants in the Carrot and Rue Families

Some plants in the carrot family (Apiaceae) and rue/citrus family (Rutaceae) contain compounds called furanocoumarins. These compounds can cause phytophotodermatitis—a rash, blistering reaction, or skin discoloration that occurs after touching the plant and then being exposed to sunlight.

Plants in the carrot family that may cause photosensitive reactions include:

• False bishop’s weed (Ammi majus)
• Hogweeds (Heracleum spp.)
• Korean angelica (Angelica gigas)
• Wild carrot (Daucus carota)
• Wild parsnip (Pastinaca sativa)

In the rue and citrus family, common culprits include:

• Bergamot orange (Citrus bergamia), including its essential oil, unless it is bergaptene-free
• Lime (Citrus aurantiifolia)
• Rue (Ruta graveolens)

Herbal medicine can be profoundly supportive, but it is not always sufficient for acute, severe, or life-threatening medical conditions. It’s important to recognize that herbs are sometimes just one part of a broader healing approach. Certain situations may call for emergency medical care, pharmaceutical medications, surgery, diagnostic testing, or other therapeutic modalities such as psychotherapy, acupuncture, physical therapy, chiropractic care, or massage therapy.

Honor your own wisdom and intuition, and learn to recognize the warning signs of potentially serious illness. Cultivate the discernment to know when conventional medical care is appropriate. Ideally, herbal medicine and conventional medicine can work alongside one another rather than in opposition.

It’s better to err on the side of caution than to let rigid beliefs about natural healing interfere with appropriate medical care. Learn the symptoms of potentially life-threatening conditions such as heart attack, stroke, heart failure, severe infection, and diabetes-related emergencies.

Trust your instincts. If something feels seriously wrong or potentially life-threatening, seek immediate medical attention.

Herbs & Natural Supplements, an Evidence-Based Guide; Volumes 1 and 2. 4th Edition by Lesley Braun and Marc Cohen. 2015.
Excellent reference covering the most common herbs and supplements. The herbal monographs feature chemical constituents and clinical applications, including any available studies on the herb. The monographs also include topical and internal side effects and pharmaceutical interactions. The books are heavily referenced and written for the clinician. Top pick if you will be purchasing only one book (or a set of books).

American Herbal Products Association’s Botanical Safety Handbook, Second Edition by Zoë Gardner and Michael McGuffin. 2013.
This is one of the most comprehensive guides to botanical safety and herb/drug interactions. It features more herbs than the other references in this list, but covers them less in-depth. The second edition was published in 2013, so it is somewhat dated.

Herb, Nutrient, and Drug Interactions: Clinical Implications and Therapeutic Strategies by Mitchell Bebel Stargrove, Jonathan Treasure, and Dwight L. McKee. 2008.
This resource covers fewer herbs–30 in total–than the other references, but fully explores the research on each herb. Much of the book is devoted to vitamins, minerals, and natural supplements. The information is somewhat dated.

Additional Sources are linked directly throughout the article.

American Herbal Products Association. Botanical Safety Handbook. 2nd ed. CRC Press; 2013.

Borrelli F, Izzo AA. Herb-drug interactions with St. John’s wort (Hypericum perforatum): An update on clinical observations. The AAPS Journal. 2009.

Burrows GE, Tyrl RJ. Toxic Plants of North America. Wiley; 2012.

Centers for Disease Control and Prevention. Drug Overdose Deaths. Published 2021.

Ernst E. Review of adulteration of Chinese herbal medicines with synthetic drugs. Journal of Internal Medicine. 2002.

Ganora L. Herbal Constituents: Foundations of Phytochemistry: A Holistic Approach for Students and Practitioners of Botanical Medicine. Herbalchem Press; 2009.

Gladstar R. Herbal Healing for Women. Simon & Schuster; 1993.

Hiller KO, Zetler G. Neuropharmacological studies on ethanol extracts of Valeriana officinalis L.: Behavioural and anticonvulsant activities. Phytotherapy Research. 1996.

Huxtable R. Human health implications of pyrrolizidine alkaloids and herbs containing them. In: Toxicants of Plant Origin. CRC Press; 1989.

Kallas J. Edible Wild Plants. Gibbs Smith; 2010.

Kesarwani K, Gupta R. Bioavailability enhancers of herbal origin: An overview. Asian Pacific Journal of Tropical Biomedicine. 2013.

Ko R, Au A. Compendium of Asian Patent Medicines. California Department of Health Services; 1997–1998.

Lazarou J, Pomeranz B, Corey P. Incidence of adverse drug reactions in hospitalized patients: A meta-analysis of prospective studies. JAMA. 1998.

McIntyre A. The Complete Herbal Tutor: The Ideal Companion for Study and Practice. Octopus Books; 2010.

Mills S, Bone K. The Essential Guide to Herbal Safety. Elsevier Health Sciences; 2005.

Neuman M, Cohen L, et al. Hepatotoxicity of pyrrolizidine alkaloids. Journal of Pharmacy and Pharmaceutical Sciences. 2015.

Palmer ME, Haller C. Adverse events associated with dietary supplements: An observational study. The Lancet. 2003.

Romm AJ. Botanical Medicine for Women’s Health. Churchill Livingstone/Elsevier; 2010.

Romm AJ. The Natural Pregnancy Book: Herbs, Nutrition, and Other Holistic Choices. Potter/Ten Speed/Harmony; 2011.

Saper R, Kales S, et al. Heavy metal content of Ayurvedic herbal medicine products. JAMA. 2004.

Shaw D, Leon C, Kolev S, Murray V. Traditional remedies and food supplements: A five-year toxicologic study (1991–1995). Drug Safety. 1997.