Drugs have the potential to interact with nutrients potentially leading to reduced therapeutic efficacy of the drug, nutritional risk or increased adverse effects of the drug. Despite significant interest in such interactions going back to over more than 40 years, the occurrence and clinical significance of many drug–nutrient interactions remains unclear. However, interactions involving drugs with a narrow therapeutic margin such as theophylline and digoxin and those that require careful blood monitoring such as warfarin are likely to be those of clinical significance. Drugs can affect nutrition as a result of changes in appetite and taste as well as having an influence on absorption or metabolism of nutrients. Moreover, foods and supplements can also interact with drugs, of which grapefruit juice and St John's wort are key examples. Significant numbers of people take both supplements and medication and are potentially at risk from interactions. Professionals, such as pharmacists, dietitians, nurses and doctors, responsible for the care of patients should therefore check whether supplements are being taken, while for researchers this is an area worthy of significant further study, particularly in the context of increasingly complex drug regimens and the plethora of new drugs.

Therapeutic drug monitoring of tricyclic antidepressants is a controversial issue. A case is reported in which a patient with recurrent depression developed asymptomatic tricyclic toxicity while on trimipramine, a tricyclic antidepressant, and diltiazem, a calcium channel blocker. A metabolic interaction involving cytochrome P450 3A (CYP3A) is postulated. It is suggested that routine therapeutic drug monitoring of tricyclic levels should be an essential part of the management of depression.