20.2 Pharmacognosy & Dietary Supplements

What Is Pharmacognosy?

Pharmacognosy is the study of medicines and bioactive substances derived from natural sources — primarily plants, but also marine organisms, microbes, and minerals. Many modern drugs originated from natural products: paclitaxel (Pacific yew), artemisinin (Artemisia annua), digoxin (foxglove), morphine (opium poppy), aspirin (willow bark salicin), and penicillin (Penicillium mould).

For the DHA Pharmacist exam, the high-yield focus is not botanical taxonomy but herb-drug interactions that change the safety or efficacy of conventional medicines.

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High-Yield Herb-Drug Interactions

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The St John's Wort Trap

St John's wort is the single most tested herbal interaction. The key principle is enzyme induction, not inhibition. Induction of CYP3A4 and P-glycoprotein lowers plasma concentrations of substrate drugs, producing therapeutic failure rather than toxicity.

The classic DHA scenarios are:

• A patient on warfarin starts St John's wort for mood → INR drops → thrombosis/stroke
• A patient on ciclosporin after renal transplant starts St John's wort → trough falls → acute rejection
• A woman on oral contraceptives starts St John's wort → breakthrough bleeding → unintended pregnancy
• A patient already on an SSRI starts St John's wort → agitation, tremor, hyperthermia → serotonin syndrome

Do not recommend St John's wort to patients on any CYP3A4/P-gp substrate, and counsel about the serotonergic interaction with antidepressants.

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Bleeding-Risk Herbs and Perioperative Cessation

Several natural products inhibit platelet function or coagulation and increase bleeding risk, especially around surgery:

• Fish oil / omega-3 fatty acids
• Vitamin E (high dose)
• Garlic
• Ginkgo biloba
• Nattokinase and other fermented soy products

For elective surgery, advise patients to stop these supplements 7-14 days preoperatively (exact interval depends on the herb and surgical bleeding risk) and to resume only after haemostasis is secure. Always confirm with the surgical team.

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Other Safety Alerts

• Kava: reports of severe hepatotoxicity have led to restrictions or bans in several countries; avoid in patients with liver disease or on other hepatotoxins (e.g. high-dose paracetamol, alcohol, statins in susceptible patients).
• Echinacea: an immunostimulant that can theoretically worsen autoimmune disease (rheumatoid arthritis, lupus, MS) and reduce efficacy of immunosuppressants.
• Valerian: sedative; additive with benzodiazepines, opioids, alcohol, and first-generation antihistamines.
• Ginseng: may lower warfarin effect (reduced INR) and alter blood glucose — diabetic patients on ginseng should monitor glucose closely.

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Regulation and Quality of Supplements

Unlike licensed medicines, most dietary supplements are regulated as foods rather than drugs in many jurisdictions, which means pre-marketing proof of efficacy, safety, and quality is far less stringent. Concentrations of active constituents can vary between batches and brands, and contamination with heavy metals, pesticides, or undisclosed pharmaceuticals (e.g. sildenafil analogues in "herbal" weight-loss or enhancement products) is a recurring safety problem. In the UAE, the DHA requires that supplements be registered and that claims be substantiated; pharmacists should stock registered products, verify batch documentation, and report suspected adverse reactions through the relevant pharmacovigilance channel.

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Focused Monographs for the Exam

• St John's wort (Hypericum perforatum): used for low mood; the major exam signal is CYP3A4/P-gp induction causing reduced levels of ciclosporin, warfarin, oral contraceptives, digoxin, theophylline, and HIV protease inhibitors, plus a serotonin syndrome risk with SSRIs, SNRIs, triptans, and MAOIs.
• Ginkgo biloba: used for cognition and circulation; platelet-activating factor inhibition → bleeding with warfarin, aspirin, NSAIDs; perioperative cessation 36 hours to 7 days depending on guidance.
• Garlic: antiplatelet and lipid-lowering claims; bleeding risk, especially with anticoagulants; stop 7-10 days pre-surgery.
• Cranberry: may inhibit CYP2C9 and raise warfarin INR; counsel monitoring rather than routine avoidance.
• Ginseng: may lower warfarin effect and lower blood glucose; diabetic patients need closer glucose monitoring.
• Kava: sedative/anxiolytic; hepatotoxicity — avoid in liver disease and pregnancy.
• Echinacea: immunostimulant; avoid in autoimmune disease and transplant recipients on immunosuppressants.
• Valerian: sedative; additive with benzodiazepines, opioids, alcohol, antihistamines.

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Counselling Pearls

• Always ask patients directly about herbal and supplement use; many do not volunteer it.
• "Natural" does not mean "safe" — many of the highest-risk products are botanicals.
• Document supplement use in the medication reconciliation.
• Counsel patients to avoid starting a new herbal product within 2 weeks of any new prescription medicine.
• High-risk patients on warfarin, transplant immunosuppressants, HIV antiretrovirals, chemotherapy, or SSRIs deserve a focused herbal-interaction review at every visit.