Herbal remedies that interact with your medicines: what to tell your doctor and why

Why do herbs interact with medicines?

The widespread belief that natural herbs are inherently safe and cannot conflict with medicines is both wrong and dangerous. Herbs contain active chemical compounds, which is precisely what makes some of them useful in certain situations. But those same compounds can interfere with the way the body processes prescribed drugs.

Most medications are metabolised through a family of liver enzymes called the cytochrome P450 (CYP450) system. Many herbs inhibit or activate these enzymes. If an herb inhibits the enzyme responsible for breaking down a particular drug, the drug accumulates to higher levels than intended and becomes more toxic. Conversely, if an herb activates those enzymes, the drug is broken down faster than it should be and its therapeutic effect falls.

A second type of interaction is direct: some herbs produce an effect similar to a specific drug, doubling the overall impact and creating an effective overdose situation. Examples include blood-sugar-lowering herbs taken alongside diabetes medications, or blood-thinning herbs taken alongside warfarin. The total effect is not one plus one: it can be three or four.

A 2000 landmark review in the Lancet by Fugh-Berman catalogued dozens of clinically significant herb-drug interactions and helped establish this field as a serious pharmacological concern. The scientific understanding has grown substantially since then, but public awareness, particularly in communities with high traditional medicine use, has not kept pace.

Herbs and anticoagulants: the most dangerous interaction

Warfarin is the drug most affected by herbs among all commonly prescribed medicines. It remains widely used across the region for treating clots, atrial fibrillation and prosthetic heart valves. The therapeutic window for warfarin is extremely narrow: a little too little fails to prevent a clot; a little too much causes serious bleeding.

A comprehensive systematic review published in the British Journal of Clinical Pharmacology in 2021 mapped dozens of documented interactions between warfarin and herbs and foods. Among the herbs that strengthen warfarin's blood-thinning effect and raise bleeding risk: garlic in large supplemental doses, Ginkgo biloba, cranberry, barberry fruit, and fish oil at high doses. Among the herbs that weaken warfarin's effect and raise clot risk: St John's Wort, ginseng and concentrated green tea extracts.

A more recent 2025 review in Seminars in Thrombosis and Hemostasis extended this analysis to newer anticoagulants such as rivaroxaban and apixaban, which are becoming more common in the region. These newer drugs are also subject to herb-drug interactions through the same liver enzyme pathways. Switching from warfarin to a newer drug does not automatically confer safety from these interactions.

Warfarin patients who monitor their INR levels may notice unexplained fluctuations despite taking the same dose. One of the commonest causes is a change in herbal use or dietary intake. Telling the supervising doctor or anticoagulation clinic about any change in herbs or diet helps explain these swings and guides dose adjustment.

Herbs that destabilise diabetes medicines

Several herbs widely used in Arab communities have documented effects on blood sugar levels, making them far from neutral when taken alongside diabetes medications. Nigella sativa (black seed) has been covered in a dedicated article and has multiple systematic reviews confirming it lowers fasting blood glucose. Fenugreek (Trigonella foenum-graecum) has a similar documented effect and is commonly used as a supplement and cooking ingredient across the region.

A person with type 2 diabetes who takes metformin, glibenclamide or insulin and then adds large amounts of black seed or fenugreek may develop hypoglycaemia. Sudden sweating, dizziness, trembling and intense fatigue may be the first signs. In more severe episodes, confusion or loss of consciousness can occur. Managing this interaction requires telling the prescribing doctor and monitoring blood glucose regularly whenever any herb is added.

The concern extends beyond sugar-lowering herbs. Some herbal slimming products contain plant stimulants that raise heart rate and blood pressure, adding another layer of risk for a diabetic patient who frequently also has hypertension and is already on multiple medications.

Liver toxicity from herbs: the less-known and more serious risk

Herb-induced liver injury is a growing medical challenge as herbal supplement use increases. A review published in the Journal of Hepatology in 2011 described how dozens of plant preparations can cause hepatitis, fibrosis and, in rare cases, acute liver failure. This is not a trivial risk; some cases have required liver transplantation.

Among the herbs most consistently associated with liver damage in published reviews: senna (Cassia senna) when overused as a laxative, kava kava which has been banned in several countries because of liver injury, comfrey which contains pyrrolizidine alkaloids known to be hepatotoxic, and various unverified herbal mixtures of Chinese or Indian origin. Products with undisclosed ingredients pose particular risk because a doctor cannot assess what the patient is actually taking.

Contamination is a separate concern. Analysis of some commercial herbal supplements has found heavy metals such as lead and arsenic, or undisclosed pharmaceutical substances added illicitly. In pharmacies and markets without strict regulatory oversight, this risk is real and documented.

Patients with chronic liver disease, whether from hepatitis B or C, fatty liver or cirrhosis, face the greatest danger from hepatotoxic herbs because their liver's capacity to process toxins is already compromised. These patients must tell their treating doctor before taking any herbal supplement, without exception.

Herbs that undermine epilepsy drugs and antidepressants

St John's Wort is used in some Arab countries and across the wider region as a folk remedy for mild depression. But it is one of the most potent activators of liver enzymes known, accelerating the breakdown of more than 50 drug types. Among the most seriously affected: hormonal contraceptives, which lose efficacy and can lead to unintended pregnancy; cyclosporine used after organ transplantation, whose levels can drop dangerously low and trigger immune rejection of the transplanted organ; and several antiretroviral medications used in HIV treatment.

Epilepsy drugs such as phenytoin and carbamazepine have narrow therapeutic windows. Concurrent use of liver-enzyme-activating herbs may lower their blood levels and increase seizure risk. Conversely, enzyme-inhibiting herbs may raise their levels to toxic concentrations, causing severe drowsiness, dizziness and potentially affecting breathing.

Selective serotonin reuptake inhibitor (SSRI) antidepressants have documented interactions with several herbs. St John's Wort itself raises serotonin levels in the brain, and combining it with an SSRI that does the same can precipitate serotonin syndrome, a medical emergency characterised by muscle rigidity, trembling, high fever and cardiac arrhythmia.

Why patients do not tell their doctors, and how to change that

A 2024 study published in Medical Science Monitor assessed nursing students in Riyadh and found significant gaps in their knowledge of herb-drug interactions despite formal medical education. If healthcare providers are not always well-informed, this underscores how unlikely a patient is to receive a thorough response when they do mention herbal use, and how important it is for the patient themselves to raise the issue clearly.

Patients do not disclose herbal use for several reasons. Some fear that the doctor will disapprove or ban the herbs. Others believe that natural products do not affect medicines. Some simply were not asked. The solution requires honest communication from the patient, and a non-judgemental response from the doctor that treats traditional remedy use as medically relevant information rather than ignorance to correct.

If you use herbs or supplements regularly, take a list to every medical appointment. Include the herb's name, its form (capsule, oil, tea, raw seed), the approximate dose and how long you have been using it. If the appointment is brief, a written list handed to the doctor or handed to the pharmacist will be received. If you have no immediate access to a doctor, a qualified pharmacist is an excellent first point of contact for interaction questions.

Other herbs worth watching in a regional context

High-dose garlic supplements have documented antiplatelet and anticoagulant properties. When taken alongside daily low-dose aspirin or warfarin, or other anticoagulants, they may increase bleeding risk, particularly in the period before and after surgical procedures. Culinary garlic in normal cooking amounts does not pose a comparable risk, but concentrated garlic capsules taken regularly warrant discussion with your doctor.

Milk thistle (Silybum marianum) is used for liver support and has some evidence in specific liver conditions. However, it interacts with liver enzymes and can alter levels of drugs including antifungals and some antivirals, both of which are commonly prescribed in the region. If you have hepatitis or are on a long-term antifungal course, check with your doctor before using it.

Ginger in large supplemental doses has antiplatelet effects and may add to the blood-thinning actions of warfarin, aspirin and non-steroidal anti-inflammatory drugs. Culinary use in cooking is not a concern at normal amounts, but concentrated capsules and extracts in people on those medications deserve a conversation with a doctor or pharmacist.

The golden rule for the informed patient

Using herbs is not inherently wrong. Many have genuine research bases or long histories of traditional use with acceptable safety records. But using them without integrating them into the full picture of your medication plan is a real mistake that can have serious consequences.

The golden rule is simple: anything you consume regularly outside of ordinary meals should be known to your doctor. Whether it is an herb, a supplement, a vitamin, a traditional medical preparation or even a combination of over-the-counter products at high doses. This information is not a judgement on your choices; it is a tool that allows your doctor to make safer decisions on your behalf.

When a doctor visit is not immediately available, a qualified pharmacist is the best first resource. Pharmacists receive specific training in drug interactions and often have more time than a doctor in a busy clinic to discuss the question properly. Asking a pharmacist whether an herb interacts with a specific medicine is a well-placed question that will receive a useful answer.

Sources

• Tan CSS, Lee SWH: Warfarin and food, herbal or dietary supplement interactions: A systematic review, British journal of clinical pharmacology, 2021
• Hazra S, Singh PA, Bajwa N et al: Safety Issues of Herb-Warfarin Interactions, Current drug metabolism, 2024
• Talasaz AH, McGonagle B, HajiQasemi M et al: Pharmacokinetic and Pharmacodynamic Interactions between Food or Herbal Products and Oral Anticoagulants: Evidence Review, Practical Recommendations, and Knowledge Gaps, Seminars in thrombosis and hemostasis, 2025
• Fugh-Berman A: Herb-drug interactions, Lancet (London, England), 2000
• Larrey D, Faure S: Herbal medicine hepatotoxicity: a new step with development of specific biomarkers, Journal of hepatology, 2011
• Alharbi K, Bakarman SS, Syed W et al: Assessment of Undergraduate Nursing Students' Understanding of Herbal Medicines and Herb-Drug Interactions in Riyadh, Saudi Arabia, Medical science monitor, 2024
• Stedman C: Herbal hepatotoxicity, Seminars in liver disease, 2002