Herbal Supplements That Interact with Common Prescription Drugs

Every year, thousands of people end up in the emergency room because they took a herbal supplement with their prescription medication-often without realizing it could be dangerous. It’s not just about taking too much. Sometimes, even a daily ginkgo capsule or a bottle of St. John’s Wort can turn a safe drug into a risky one. The problem isn’t the supplement itself. It’s the hidden interaction. And most people have no idea it’s happening.

Why Herbal Supplements Can Be Risky with Prescription Drugs

Herbal supplements aren’t regulated like prescription drugs. That means they don’t have to prove safety or effectiveness before hitting store shelves. Under the Dietary Supplement Health and Education Act of 1994, companies can sell anything labeled as a ‘dietary supplement’ without FDA approval. The agency can only step in after someone gets hurt. That’s why you’ll find products claiming to boost memory, lower blood pressure, or lift your mood-with no warning labels about what they might do when mixed with your heart medication or antidepressant.

The body processes herbs and drugs through the same pathways. Many herbs affect liver enzymes-especially CYP3A4 and CYP2C9-that break down medications. If an herb slows down these enzymes, your drug builds up in your system and can cause toxicity. If it speeds them up, your drug gets cleared too fast and stops working. This isn’t theory. It’s been proven in clinical trials.

Top 5 Dangerous Herbal Interactions

Some herbs are far more likely to cause harm than others. Here are the five with the strongest evidence and the most serious outcomes:

• St. John’s Wort and antidepressants: This herb is one of the most dangerous combinations. It can slash the levels of SSRIs like sertraline and fluoxetine by 20-40%, making them useless. Worse, it can trigger serotonin syndrome-a potentially fatal condition marked by high fever, muscle rigidity, confusion, and rapid heartbeat. The American Academy of Family Physicians says it should be avoided entirely with any antidepressant.
• Ginkgo biloba and blood thinners: Ginkgo is popular for memory and circulation, but it increases bleeding risk. When taken with warfarin, apixaban, or aspirin, it can cause internal bleeding. Mayo Clinic recorded 23 major hemorrhage cases between 2010 and 2020, including three deaths. INR levels can spike above 8.0, far beyond the safe range of 2.0-3.0.
• Garlic supplements and HIV medications: Garlic reduces the concentration of saquinavir, an HIV protease inhibitor, by 51%. That’s enough to let the virus rebound. It also interacts with blood thinners and can increase bleeding risk.
• Goldenseal and many prescription drugs: Goldenseal blocks the CYP3A4 enzyme, which handles about half of all medications. It can raise levels of statins, sedatives, and immunosuppressants like cyclosporine to toxic levels. One study showed a 40% drop in midazolam clearance-meaning sedatives last longer and can cause dangerous drowsiness.
• Coenzyme Q10 and warfarin: Many people take CoQ10 for heart health or statin side effects, but it can reduce warfarin’s effect by 25-30%. That means your blood clots faster. Weekly INR checks are needed if you’re taking both.

What About the ‘Safe’ Herbs?

Not all herbs are equally risky. Some, like milk thistle, saw palmetto, and American ginseng, have little to no documented interaction with common drugs. But that doesn’t mean they’re harmless. The evidence is often weak because most studies are small, short-term, or based on lab results-not real patients.

Cranberry juice is a good example. For years, people were told it could boost warfarin’s effect and cause bleeding. But recent reviews show mixed results. Some studies found no effect. Others saw INR increases in just 12% of users. The inconsistency means doctors can’t give a blanket warning-but they also can’t say it’s safe. If you drink cranberry juice daily and take warfarin, get your INR checked more often.

Even herbs marketed as ‘natural’ or ‘gentle’ can be risky. Chamomile, for example, is often used for sleep or anxiety. But it can interact with sedatives and blood thinners. And if you’re on a beta-blocker for high blood pressure, hawthorn can push your blood pressure too low-leading to dizziness, fainting, or falls.

Who’s Most at Risk?

Older adults are the most vulnerable. Nearly 70% of people over 65 take at least one supplement, and more than half are on two or more prescription drugs. Many don’t realize their supplements are part of their medication list. A University of Michigan study found 76% of supplement users believed ‘natural means safe.’ Only 25% tell their doctor about what they’re taking.

That’s a problem because doctors don’t ask. Most primary care visits focus on symptoms, not supplements. Pharmacists are better trained to catch these interactions, but they’re not always consulted. In the Medicare population, 14.4% of seniors used herbal supplements alongside prescription drugs. Of those, nearly 400 cases involved dangerous bleeding risks from ginkgo, garlic, or ginseng combined with blood thinners.

People on immunosuppressants after organ transplants are also at extreme risk. St. John’s Wort can drop cyclosporine levels by 50-60% in just two weeks. That’s enough to cause organ rejection. One patient lost a kidney transplant because they started taking St. John’s Wort for ‘depression’ without telling their transplant team.

What You Should Do

If you take any prescription medication, here’s what you need to do:

1. Make a complete list of everything you take: pills, capsules, teas, tinctures, and even topical creams. Don’t leave out ‘natural’ products.
2. Bring it to every doctor and pharmacist visit. Don’t wait to be asked. Say: ‘I take these supplements. Are they safe with my meds?’
3. Check high-risk products. If your supplement contains ginkgo, St. John’s Wort, garlic, goldenseal, or licorice, assume it’s risky until proven otherwise.
4. Watch for warning signs. Unexplained bruising, nosebleeds, dizziness, confusion, rapid heartbeat, or sudden mood changes could signal a dangerous interaction.
5. Use trusted resources. The National Center for Complementary and Integrative Health (NCCIH) has a free, updated herb-drug interaction checker. Memorial Sloan Kettering’s database is also reliable.

Pharmacists can run your list through Stockley’s Herbal Interactions database. It flags interactions by severity: ‘Life-threatening,’ ‘Significant,’ or ‘Moderate.’ Ask them to run your meds and supplements through it. It takes less than five minutes.

The Bigger Problem: Lack of Evidence and Regulation

Here’s the uncomfortable truth: We don’t know enough. Only 15% of documented herb-drug interactions have strong evidence from randomized trials. The rest come from case reports, animal studies, or lab tests. That means some warnings are based on a single patient’s story-not solid science.

But that doesn’t mean you should ignore them. When the stakes are bleeding, organ rejection, or serotonin syndrome, you don’t wait for perfect data. You assume risk until proven safe.

The FDA doesn’t test supplements before they’re sold. Companies don’t have to report side effects unless they’re severe. An estimated 95% of adverse events go unreported. In 2022, the FDA warned 17 companies for slipping actual drugs-like sildenafil or steroids-into their ‘natural’ products. That’s not a supplement issue. That’s a fraud issue.

What’s Changing?

Things are slowly improving. The NCCIH updated its interaction database in January 2024, adding new findings like green tea reducing the effectiveness of the cancer drug bortezomib. Epic Systems, the electronic health record platform used by most U.S. hospitals, plans to integrate NCCIH data into medication reconciliation by late 2025. That means when you’re admitted, your list of supplements could automatically flag interactions.

But until then, the responsibility falls on you. No one else is checking. No one else is asking. If you’re on warfarin, antidepressants, statins, or immunosuppressants, your herbal supplement might be quietly undoing your treatment-or making you sick.

‘Natural’ doesn’t mean safe. And ‘over-the-counter’ doesn’t mean harmless. The only way to stay safe is to talk to your doctor-every time.