Herb	Conventional drug	Potential problem
Echinacea used for >8 weeks	Anabolic steroids, methotrexate, amiodarone, ketoconazole	Hepatotoxicity
Feverfew	Non-steroidal anti-inflammatory drugs	Inhibition of herbal effect
Feverfew, garlic, ginseng, gingko, ginger	Warfarin	Altered bleeding time
Ginseng	Phenelzine sulphate	Headache, tremulousness, manic episodes
Ginseng	Oestrogens, corticosteroids	Additive effects
St John’s wort	Monoamine oxidase inhibitor and serotonin reuptake inhibitor antidepressants	Mechanism of herbal effect uncertain. Insufficient evidence of safety with concomitant use—therefore not advised
Valerian	Barbiturates	Additive effects, excessive sedation
Kyushin, liquorice, plantain, uzara root, hawthorn, ginseng	Digoxin	Interference with pharmacodynamics and drug level monitoring
Evening primrose oil, borage	Anticonvulsants	Lowered seizure threshold
Shankapulshpi (Ayurvedic preparation)	Phenytoin	Reduced drug levels, inhibition of drug effect
Kava	Benzodiazepines	Additive sedative effects, coma
Echinacea, zinc (immunostimulants)	Immunosuppressants (such as corticosteroids, cyclosporin)	Antagonistic effects
St John’s wort, saw palmetto	Iron	Tannic acid content of herbs may limit iron absorption
Kelp	Thyroxine	Iodine content of herb may interfere with thyroid replacement
Liquorice	Spironolactone	Antagonism of diuretic effect
Karela, ginseng	Insulin, sulphonylureas, biguanides	Altered glucose concentrations. These herbs should not be prescribed in diabetic patients