Supplements will not generally stop medicines from working. However, there are interactions between nutritional status and the action of some medicines.
Certain medicines influence the absorption or metabolism of some nutrients. For example, some anti-ulcer agents reduce the absorption of vitamin B12 and the anticonvulsant drug phenytoin increases the breakdown of folic acid.
Conversely, certain nutrients may influence the action of medicines; vitamin K reduces the clotting effect of the blood which means that the effects of anti-coagulant medications like warfarin will be increased leading to a greater risk of bleeding.
In addition, some herbal remedies may influence the efficacy or toxicity of some prescription medicines; for example St. John’s Wort (hypericum) reduces the efficacy of the contraceptive pill.
People should always read the product indications carefully before taking a supplement, and if still uncertain, consult a pharmacist or the doctor who has prescribed the medicine.