Herb-Drug Interactions: Fact Fiction or Somewhere Between

One of the biggest fears practicing herbalists have is the possibility of harming someone when giving medicinal herbs. But when a client is taking one, two, or ten medications, the risk of harming them with herbs dramatically goes up . . . or does it?

Herb-drug interactions—is it fact, fiction, or somewhere in between? This is an important topic to discuss since so many people today take prescription drugs, sometimes multiple different ones each day. The fear of harming someone makes a lot of herbalists feel intimidated to work with those taking medications. While the fear is warranted, do we really need to be overly cautious about prescribing herbs to people taking medications? The answer to that is, like herb-drug interactions, a bit complicated. 

Let’s start with the basics: What exactly are herb-drug interactions? Herb-drug interactions occur when the chemistry of a medication and an herb interact, influencing the effect of the drug. Although we tend to think of these interactions as negative, sometimes they can even be positive. There are three main ways plants interact with medications: synergistic interactions, protective interactions, and contraindications. Let’s explore each one. 

Synergistic Interactions

This is an additive action where the herb enhances the effect of the drug. These interactions can be helpful or harmful. For example, if someone is taking blood pressure medication and wants to use an herb that also lowers blood pressure, they might need to reduce their medication dosage (under a doctor’s supervision). Otherwise, the herb can make the drug’s effect too strong and lead to issues.  Another example is using an herbal hypnotic for pain management alongside a muscle relaxant like cyclobenzaprine. This combination can enhance musculoskeletal relaxation, making the drug (and herb) more effective since both are achieving the same end result, albeit via different biochemical pathways. Another example is berberine-containing plants like Oregon Grape (Berberis aquifolium), Coptis (Coptis chinensis), and Goldenseal (Hydrastis canadensis), which can increase the efficacy of antibiotics, making this combination beneficial for people with antibiotic resistance.

In this way, synergistic interactions can help someone lower the dosage of their prescription medication. However, this is a decision that can only be made between client and doctor. To reiterate this critically important point, no herbalist should ever tell a client to adjust the frequency or dosage of their medications!

Protective Interactions

Many medications can be toxic over the long term, and most, if not all, create side effects. Some herbs can protect the body from these toxic side effects. For example, Nettle seed (Urtica dioica) can protect the kidneys from nephrotoxicity and Milk Thistle (Silybum marianum) is known for its hepatoprotective properties, shielding the liver from damage caused by excessive acetaminophen and other harsh substances. In many instances where someone needs to be on their medication, herbs can be used to help mitigate adverse side effects. 

Contraindications

These are herbs that negatively interfere with a drug. A famous example is St. John’s wort (Hypericum perforatum) and how it interacts with selective serotonin reuptake inhibitors (SSRIs), a type of antidepressant. This interaction can potentially lead to serotonin syndrome, which is a serious condition, making it a harmful herb for people taking SSRIs to take. This interaction is actually due to St. John’s Wort having an influence on specific enzyme systems in the liver that are major detoxification pathways for medications (and many other substances in the body). In essence, it affects the metabolism of the medications, in some cases prolonging their influence, and in others reducing it. This is very important to consider for medications that have what are called a “narrow therapeutic window,” meaning the blood levels must be maintained within a certain narrow parameter to be effective and not ineffective or toxic. 

Understanding herb-drug interactions is critical for providing safe and effective care. And while the potential for harm is a valid concern, it’s important to recognize the possible benefits and to approach this topic with a balanced perspective. By learning about synergistic, protective, and contraindicated interactions, we can better navigate the complexities of combining herbs with medications. Remember, the goal is to enhance the well-being of those coming to us for support, and with the right knowledge, we can confidently support those who are taking medications.

Navigating Herb-Drug Interactions: What You Need to Know

Although it may feel daunting to learn about herb-drug interactions, understanding this topic is necessary if you want to ensure your herbal recommendations are safe and effective. Herbs often influence drugs either through their influence on metabolism or absorption. Let’s delve into these primary ways so you can learn what you need to be aware of to safely navigate these complexities.

Metabolism

When we think about how drugs work, we tend to focus on their pharmacokinetics and pharmacodynamics. Pharmacokinetics involves how the drug is absorbed, distributed, transformed, circulated, and cleared from the body. Pharmacodynamics looks at what the drug does when it reaches its binding site—how it changes physiology and biochemistry to bring about a particular effect. Many herbs can influence drug metabolism, affecting these processes.

For instance, herbs can either speed up or slow down the metabolism of a drug. Speeding up metabolism can decrease drug levels in the bloodstream while slowing it can prolong the drug’s effect. These interactions are especially important for drugs with a narrow therapeutic index, meaning they need to stay within a specific blood level range to be effective and safe. An example of such a drug is warfarin, used for cardiovascular disease due to its anticoagulant effects.

Known for treating puncture wounds and depression, St. John’s wort significantly impacts the liver, specifically phase one liver detoxification. Resultantly, it seriously impacts drug metabolism by influencing cytochrome P450 enzymes, which detoxify and prepare substances for phase two detoxification and excretion. By affecting these pathways, St. John’s wort can reduce the blood levels of certain drugs, making them less effective, or it can increase plasma levels, causing further issues. This is why this herb has more contraindications than others. 

Absorption

Another way herbs can cause contraindications is by affecting drug absorption in the gastrointestinal tract. Some herbs enhance absorption, while others decrease or even prevent it entirely. For example, Prickly Ash (Zanthoxylum clava-herculis) is a pungent and aromatic circulatory herb that’s a gastrointestinal stimulant and can enhance the absorption of medications. This can alter the effectiveness of drugs that need to be absorbed at a specific rate.

Similarly, citrus fruits enhance absorption, which can be problematic if your doctor prescribes a drug that needs to be absorbed slowly or at a certain rate. When absorption increases, the drug will be absorbed faster, which can affect the plasma levels of that drug and lead to issues. 

Another way herbs can cause contraindications involves those containing high levels of mucilaginous polysaccharides, or demulcents, such as Slippery Elm (Ulmus rubra) and Marshmallow (Althaea officinalis L.). These mucilaginous polysaccharides can significantly prevent the absorption of not only certain drugs but also nutrients and minerals. Therefore, it’s important to take these herbs several hours away from medication to avoid impacting their absorption.

Herbs that contain tannins and are very astringent, such as Oak bark (Quercus L.), Witch Hazel (Hamamelis virginiana) , and Lady’s Mantle (Alchemilla vulgaris) can also affect absorption, particularly of minerals. Tannins precipitate alkaloids, leading to potential interactions between astringent herbs and alkaloidal medications or even alkaloidal herbs. Generally it’s best to take your astringents separately from everything else, especially any nutrients or vitamins as they will affect the absorption. 

These are some ways I consider herb-drug interactions. Not all interactions are negative—some can even be helpful. The bottom line is that herb-drug interactions fall somewhere between fact and fiction. While it’s true that some herbs interact with certain drugs, the fiction arises when blanket statements are made, such as, all herbs interact negatively with all drugs. 

Debunking Herb-Drug Interaction Myths

I find that some medical professionals, especially medical doctors unfamiliar with herbal medicine tend to err on the side of caution by recommending avoiding herbs and supplements altogether when on medication to protect their patients. Although the intention is good, this conservative approach can be unnecessarily restrictive since not all herbal medicines negatively interact with drugs. In fact, serious herb-drug interactions are relatively rare.

While documented cases exist, they are relatively uncommon. Occurring more often than herb-drug interactions are drug-drug interactions. These are typically much more severe but are often overlooked. When we consider that some patients are on 2-5 or even up to 10-15 medications, it’s almost impossible to know what’s interacting with what. Although plants are biochemically complex, they don’t have the same potency as drugs, which are isolated and targeted. Overall, herbs are much milder compared to prescription drugs and are no match for their strength. This is why true contraindications are rare. 

Many herb-drug interactions you hear about are theoretical. For example, you may read that an herb interacts with a drug based on research, but if you look closer, you’ll see it’s a theoretical interaction—not a documented case where something happened. In reality, it’s quite unusual to find well-documented cases of consistent herb-drug interactions. 

Another research flaw is that many herb-drug interactions are based on research conducted on isolated constituents in plants. These studies often use doses much larger than is possible to obtain from the whole plant, whether by capsule, decoction, infusion, or tincture. Research based on individual compounds is flawed since isolated constituents work differently than whole plants do. 

Different compounds in a plant can affect absorption, metabolism, excretion, and biochemical transformations within the body, which isolated constituent research often overlooks. This is why it’s critical to understand whether an interaction is happening in response to the whole plant or an isolated constituent. It’s also important to look at the dose used in studies and see whether it reflects the reality of what an herbalist would safely give someone.

When an actual case report does surface about a potential herb-drug interaction, a whole new level of reliability of the information should be called into question (or at least further investigated). It’s easy to attribute an herb-drug interaction when there’s some type of response happening between an herb and drug but it’s critical to assess other factors, such as additional medications, contributing health conditions, or even the authenticity of the herb used. For example, years ago there were reports of hepatotoxicity occurring from Coltsfoot (Tussilago farfara). When the case was investigated, it was discovered that it was adulterated with Butterbur (Petasites hybridus), an herb with high levels of pyrrolizidine alkaloids, which harm the liver. 

Herb-drug interactions do occur—but the reality is that these interactions are nuanced. Some are positive and some are negative. It’s a fact that interactions happen, but it’s fiction to believe they happen all the time or occur between all herbs and drugs. 

If you’re an herbalist seeing clients and doing clinical work, you need to be aware of all of the nuances and factors that go into herb-drug interactions. If you are working with people taking prescription drugs, my top recommendations are the following:

1. List every drug your client is taking and record when they started taking it, how long they’ve been taking it for, their dosage, and what it’s for. This is an important question to ask since people can take prescription drugs for conditions not commonly associated with them, such as antidepressants for migraines. 
2. Once you have a list, research every single medication to understand how they work pharmacologically. How are they metabolized, do they have a prolonged half-life or a narrow therapeutic index? Research their side effects and get a grasp of what the drug does in the body. 
3. Comparing that list of side effects to the primary complaints of the client can oftentimes lend deep insight into the case for both practitioner and client alike. It is not uncommon for primary complaints to be the side effects of one or multiple medications. 

Once you gather this information, create your herbal formula and cross reference each herb with their medications to ensure there are no contraindications. Typically there are none, but you never want to assume. Always do your due diligence and research each one to be on the safe side. 

My Top Book Recommendations

My favorite resource for researching herb-drug contraindications is the book “Herb Contraindications and Drug Interactions” by Francis Brinker. This was the textbook we used during my series of herb-drug interactions classes at Bastyr University. Other excellent ones include “Stockley’s Herbal Medicines Interactions” (available in PDF form for free online), “Herb, Nutrient, and Drug Interactions: Clinical Implications and Therapeutic Strategies” by Mitchell Bebel Stargrove, Jonathan Treasure, and Dwight McKee, “American Herbal Products Association’s Botanical Safety Handbook” by Zoë Gardner and Michael McGuffin, “The Essential Guide to Herbal Safety” by  Simon Mills and Kerry Bone, and it’s never a bad idea to have a copy of the most updated PDR (Physicians’ Desk Reference).

I recommend you avoid getting your research from Google since there’s a lot of misinformation. Additionally, many sites are overly conservative, suggesting you never take any herb with any drug ever. The best way to find reliable information is to derive it from people who scoured the research and sorted fact from fiction for you. 

Returning to the essential question: Herb-drug interactions—fact or fiction? The answer lies somewhere in between. There are factual components, but there are fictitious ones too. Although there are legitimate reasons to be concerned, fear-mongering distorts reality and can make you feel more fearful than you need to be. 

Excessive precaution around herb-drug interactions comes from a good place, one of wanting to keep people safe. However, by following these steps and relying on trustworthy information, you can be smart, cautious, and confident in helping your clients safely receive the help they need—including those taking medications.