Consumers often perceive herbal preparations as "natural" and free from side effects. The most common adverse effect related to herbal preparations is hepatic toxicity, possibly due to the presence of pyrrolizidine alkaloids.

Most cases of toxicity may go unreported, either patients may not recognise herbs as a cause of an adverse reaction or they were not asked by medical professionals. This section deals with herbs based on their popular use and the potential for adverse effects. It is beyond the scope of this article to provide a comprehensive list of herbal remedies.

Kelp contains iodine and patients with thyroid disorders should avoid it. Caution is also needed in patients on amiodarone (Cordarone™) which is known to cause hypo- or hyperthyroidism. Kelp can concentrate heavy metals. Inadequate water intake with kelp can result in gastrointestinal obstruction.

Agnus castus is claimed to be a remedy for females with menstrual and menopausal disorders. Pharmacologically it acts on the pituitary gland to increase luteinising hormone (LH) and prolactin levels while decreasing follicle stimulating hormone (FSH) production. Interactions may occur in females taking oral contraceptives, hormone replacement therap0y and other forms of sex hormones.

Garlic has possible effects in lowering cholesterol, preventing cancer and inhibiting microbes. It has substantial antiplatelet effects. Care should be taken in patients on anticoagulation therapy and/or those about to undergo invasive surgery.

Valerian officianalis (Valerian) has sedative and tranquillising properties. It was implicated in causing hepatic toxicity. The alkylating properties of Valerian in vivo is a concern although carcinogenicity is not demonstrated in vivo. Whilst it may be an alternative to benzodiazepines, valerian should be avoided in patients with liver disease, those on drugs that are potentially hepatotoxic and drugs that require hepatic elimination. It should be avoided in pregnancy.

Ginseng has been primarily perceived as a tonic. Long term users of large daily doses may result in hypertension, insomnia and morning diarrhoea. Mastalgia and vaginal bleeding have also been reported. It should not be consumed by children, pregnant women and those on antihypertensives, hypoglycaemics and steroid therapy.

Gingko biloba (maidenhair tree) is used as a herb to slow the ageing process, perhaps due to the presence of the flavonoids, thought to be beneficial in patients with cerebral insufficiency. It is a platelet-activating factor (PAF) antagonist. Gingko can cause headaches, gastrointestinal disturbances and skin reactions. No drug interactions have been reported so far.

Dandelion is often used in preparations for arthritis, premenstrual syndrome and weight loss. Its effect is primarily diuretic; patients on conventional diuretics should be advised against its use. It also has a mild laxative effect and interferes with bile blow.

Alfafa is claimed to help in arthritis. It can induce a systemic lupus erythematosus (SLE)-like reaction.

Table 11: Problems with some herbal preparations:

Large doses of vitamin E (more than 400 units daily) may block the effects of vitamin K-dependent clotting factors. Patients on anticoagulants who self initiate vitamin E intake should be monotired for bleeding. The overall effect of folic acid intake on serum phenytoin concentration is uncertain although there has been some reports indicating loss of seizure control in patients on phenytoin therapy. It has been postulated that folic acid may increase the metabolism of phenytoin. Vitamin A supplements should be avoided in individuals on isotretinoin therapy since there may be a risk of hypervitaminosis A. It should also be avoided in pregnancy because of its teratogenic properties. Large doses of vitamin C may interfere with some laboratory and urine tests since it is a strong reducing agent.

Vitamin A should be avoided in pregnancy, in patients taking oral retinoids

Large doses of vitamin E has anticoagulation effect

Folic acid may cause loss of seizure control in patients taking antiepileptics

Vitamin C may interfere with laboratory tests

Grapefruit juice has been reported to increase the effects of cyclosporin (Neoral™), the dihydropyridine calcium channel blockers such as nifedipidine (Adalat™) and felodipine (Agon™, Plendil™) and terfenadine (Teldane™). Naringin, a bioflavanoid which gives grapefuit juice its characteristic bitter taste, is metabolised in humans to naringenin, the substance thought to be an inhibitor of cytochrome P450 enzymes. Although the clinical significance of these interactions are not well documented, caution should be exercised with cyclosporin (since it has a narrow therapeutic index) and terfenadine (especially in patients with cardiac abnormalities).

In conclusion, most patients do not regard non-prescription drugs as "medicines" and are less likely to disclose their use to GPs. Herbal remedies are perceived to be "natural" and free from side effects. Physicians may ask patients to provide a list of all medications consumed routinely. Community pharmacists have also been encouraged to provide a comprehensive medication list to clients as they are in a much better position to monitor non-prescription drug use.


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