Complementary and Alternative Medicines: Herbal Products, Vitamins and Interactions

Complementary and Alternative Medicines in NZ

Complementary and alternative medicines (CAMs) encompass a broad range of products and practices used alongside or instead of conventional medicine. In New Zealand, herbal medicines and dietary supplements are regulated under the Dietary Supplement Regulations 1985, which set standards for safety and labelling but do not require evidence of efficacy before sale. This contrasts markedly with the rigorous approval pathway required for conventional medicines under the Medicines Act 1981.

Pharmacists are frequently the first point of contact for patients seeking advice on CAMs, making it essential to have sufficient knowledge to identify clinically significant risks, especially herb-drug interactions.

Regulatory Framework for CAMs in NZ

Dietary supplements (including vitamins, minerals, herbal products, and sports supplements) must comply with the Dietary Supplement Regulations 1985. They cannot claim to treat, prevent, or cure disease (therapeutic claims require Medsafe approval as a medicine). Labelling must include ingredients and their quantities, directions for use, and storage requirements. There is no pre-market efficacy requirement — products must merely be safe at stated doses.

Natural health products sold in New Zealand may also fall under the Natural Health and Supplementary Products Act 2022, which replaces aspects of the older regulatory framework and introduces a tiered framework with a permitted ingredients list. Pharmacists must be familiar with both frameworks.

Clinically Significant Herb-Drug Interactions

The most important herb-drug interactions in NZ pharmacy practice include:

St John's Wort (Hypericum perforatum) is a potent inducer of cytochrome P450 enzymes (CYP3A4, CYP2C9) and P-glycoprotein. It can significantly reduce plasma concentrations of: oral contraceptives (risk of unintended pregnancy), antiretroviral drugs (risk of treatment failure), warfarin (risk of thrombosis from reduced INR), ciclosporin (risk of organ rejection), digoxin, and some antiepileptics. St John's Wort is contraindicated in patients taking any of these medicines.

Echinacea has potential immunostimulant properties and may theoretically antagonise immunosuppressant therapy (ciclosporin, tacrolimus). Evidence is limited but caution is warranted in transplant patients.

Ginkgo biloba inhibits platelet-activating factor and may increase bleeding risk when combined with anticoagulants (warfarin, heparin), antiplatelet agents (aspirin, clopidogrel), or NSAIDs.

Garlic supplements have antiplatelet and fibrinolytic activity and can enhance the effect of anticoagulants. High-dose garlic may lower blood pressure — additive effect with antihypertensives.

Kava (Piper methysticum) is a traditional Pacific plant medicine widely used in NZ. Kava preparations may cause additive CNS depression with benzodiazepines, alcohol, and sedating antihistamines. Rare hepatotoxicity has been reported — kava is contraindicated in pre-existing liver disease.

High-dose vitamins: Vitamin E at doses >400 IU/day may increase bleeding risk with anticoagulants. Vitamin K-containing supplements (e.g., multivitamins) can antagonise warfarin — patients should be advised to take a consistent daily amount. Vitamin D and calcium together at high doses may increase hypercalcaemia risk in patients on thiazide diuretics.

Pharmacist's Role in CAM Counselling

The pharmacist must take a complete medication history that includes all CAMs, herbal products, vitamins, supplements, and traditional medicines. Patients may not volunteer this information unless specifically asked. The pharmacist should: assess therapeutic need (is the CAM likely to help?), identify known interactions with prescribed medicines, advise on quality (choose products from reputable manufacturers with standardised extracts), warn about lack of efficacy evidence for many products, and counsel about the importance of disclosing CAM use to prescribers.