When we learn from a patient, clinician, or medical record that a drug has been discontinued, it is logical to ask why. The drug may no longer be needed; it may not have produced the desired effect; it may have produced an adverse reaction; a better drug may be available to replace the original drug. The patient may have discontinued the drug because he or she could not see why it was necessary; or the patient may have discontinued the drug because of unpleasant side effects. A drug may not work because its absorption is reduced by physical or chemical interaction with another drug or a food component. It may also not work because the patient's metabolism is speeded up or in­ hibited to an extent such that the desired duration of drug action is not obtained. Such an effect may be related to a change in diet. Side effects may be related to consumption of specific foods or bev­ erages or to an overall change in nutritional status. Drug-food and drug-alcohol incompatibility reactions are frequent but are avoidable if a patient is warned of their possible occurrence. Drugs may also produce nutritional deficiencies, especially in a patient whose diet is marginal in those nutrients depleted by the particular drug. Careful prescribing practices together with appropriate nutrient supplements will serve to reduce the risk of these incompatibilities.