Since ancient times, herbs have been used to help tissues heal, kill pathogens, improve nutrient status and boost mood. Like any powerful medicine, herbs have to be taken the right way and in the right amount to work effectively and—most importantly—to do no harm. 

In general, an herb is defined as any plant that can be used as a medicine, spice, or perfume. In this article, we’ll review the medicinal herbs you should use with caution, particularly if you’re taking other pharmaceutical medications.

A good rule of thumb that I’ll mention several times throughout this article: You should always let your doctor know when you start a new supplement, and you should always ask your herbal medicine practitioner or naturopathic doctor before choosing a new herb to add to your regimen. 

As you’ll read below, many herbs available in our grocery stores or online can actually be dangerous when combined with pharmaceuticals or when taken by someone with a pre-existing health condition. Doctors, herbalists, and even nutritionists get excellent training on how to help you safely use herbs. Take advantage of their expertise to keep yourself as healthy as possible! 

1. Licorice Root and Cardiac Medications

Licorice root has many excellent medicinal properties. It’s anti-inflammatory, adaptogenic, and even antimicrobial. It also increases blood pressure by encouraging salt retention, so it can be a great part of a treatment protocol for people with hypotension (also known as low blood pressure) but is dangerous for people with hypertension (high blood pressure). 

Licorice root is a common ingredient in over-the-counter and grocery store teas, tinctures, and medications, and it’s often contained in blends that say “cold and flu,” “anti-inflammatory,” and “detox.” It is sometimes listed on the ingredients label by its Latin name, glycyrrhiza glabra, rather than simply “licorice,” which can make it tricky to spot. If you’ve got high blood pressure, it’s worth the extra effort to check for licorice or glycyrrhiza when you’re shopping and check out the teas in your house to make sure this ingredient is not on the list. This is because licorice can interfere with and decrease the effectiveness of diuretics and other cardiac medications, and put you at risk for cardiovascular events.

You certainly should not knowingly ingest licorice root if you have hypertension, cardiovascular disease, or issues retaining fluid. The only exception to this rule is a form of licorice called DGL or deglycyrrhized licorice. This type of licorice is processed to remove glycyrrhizin—the main ingredient in licorice that causes salt retention. If you’re someone with hypertension who still wants to take licorice, DGL could be a safe option for you. As always, ask your doctor before adding any herbal medicine to your regimen, particularly if you’re already taking pharmaceutical medications. 

2. St. John’s Wort and Antidepressants

St. John’s wort is a wonderful plant with antiviral and mood-boosting properties, but it comes with two important interaction warnings. First, St. John’s wort can put you at risk for serotonin syndrome if taken along with other serotonin or MAO receptor modulating medications (antidepressants, anti-seizure medicines, anti-diarrhea medicines, and more). Serotonin syndrome can result in symptoms like seizures, fever, muscle rigidity, tremors, gastric distress, and even death if it’s not treated appropriately. Additionally, St. John’s wort can increase the metabolism of chemical birth control pills, making them less effective. This can result in breakthrough bleeding, hormonal issues, and a decrease in birth control’s effectiveness as a contraceptive. If you were hoping to use St. John’s wort to treat your mood or to help prevent illness, consider asking your doctor to check your levels of vitamin D as well as iron and vitamin B12. Evidence shows that these are safe to take with most medications, and can often improve mood and immune system function safely if it turns out you’ve got a deficiency.

3. Ginkgo Biloba and Aspirin

Ginkgo biloba has been studied for its positive effects on mental health, cognition, and cardiovascular health. It can, however, interfere with drugs like Warfarin and aspirin and has been linked to excessive bleeding when taken with these two drugs. Ask your doctor before taking it, especially if you’re using aspirin or warfarin or have a cardiovascular condition or bleeding/clotting disorder, and choose other nootropics and vascular tonics like bacopa or berries instead that won’t interfere with your medications.

4. Herbal Laxatives: Senna and Cascara

Herbal laxatives like senna and cascara sagrada are often included in “liver support,” “kidney support,” and “colon support” herbal blends that can be bought over-the-counter. These herbal laxatives can be used short-term to help move food through the digestive system quickly, but they should never be used for more than three days at a time, and not consistently for more than two weeks. This is because herbal laxatives—like all laxatives—work in part by forcing the colon to excrete more electrolytes than normal. Over time, this can lead to dehydration and electrolyte disturbances that impact electrical signaling in our hearts, brains, and other organs. Laxatives can also impact the absorption of oral medications and may reduce the effectiveness of medications aimed at controlling gastrointestinal symptoms. If you’re constipated, ask your doctor if a supplement like magnesium or vitamin C could help you go to the bathroom without impacting your other medications or putting you at risk for dehydration. If you’re looking to detox your liver safely, adding more fiber and water to your diet is a good starting point. 

5. Central Nervous System Modulating Low-Dose Herbs

These types of herbs are known as “low dose herbs” because they have to be dispensed at incredibly low doses to avoid causing serious harm or death. They were mostly used as pain-relieving medicines at a time in history before the invention of pharmaceutical medications and before we completely understood their chemistry. When it comes to these herbs, you should work with an incredibly well-trained professional to determine the ideal dose you should be taking or consider another alternative that’s less risky. The low-dose herb list includes (at a minimum):

Because most of these herbs affect the nervous system, they can interact and interfere with pharmaceutical drugs that are meant to affect mood, pain, seizures, and more. You should never start them on your own without consulting a health professional.

6. Liver-Taxing Herbs

All drugs and medicines are metabolized by the liver. Some herbal medicines can cause damage to the liver if taken in excessive amounts, for too long, or if they’re contaminated with mold, bacteria, or heavy metals due to poor manufacturing and quality control processes.

Some well-known herbs that have been implicated in cases of liver damage include

These herbs can be used safely, but you’d need professional guidance to do so. Make sure to ask your healthcare provider before using them and get their advice about what form and how much to take. 

There are many more herbs that could make this list. My advice is to avoid taking herbs multiple times a day or for weeks in a row without guidance because any herb taken in excess can damage the liver. Check out the search term “herb-induced liver injury” in PubMed to see more on this topic.

7. A Special Mention: Grapefruit 

While grapefruit isn’t technically an herb, I mention it here because grapefruit juice is consumed as a drink by over 20% of American households, and even more worldwide. It can interact with liver and intestinal enzymes to increase concentrations of pharmaceuticals in the bloodstream.  

When this happens, a person can wind up over-medicated or under-medicated compared to normal, even if they’re still taking their normal dose of medication. Consume grapefruit several hours away from pharmaceutical medications to reduce the likelihood of this interaction and ask your doctor if it’s okay to mix your specific prescription with grapefruit and/or its juice.


Despite the interactions mentioned in this article, there are thousands of safe and effective herbal medicines that you can use with the right guidance and technique to add to your health. Your takeaway should be to always let your doctor know if you’re taking herbal medications or supplements and to get a health professional’s guidance and advice when it comes to choosing an herb that’s right for you. Herbs are medicines and we should treat them that way to get the safest, best results from them.


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