If the diagnosis is wrong, incorrect prescribing will occur. It also occurs when a physician is forget and not aware that genetic and environtmental factors or the disease per se may alter the patient's response to a drug. For example; (1) Cigarette-smoking may markedly accelarate the rate elimination of a variety of drugs, (2) Patients with the nephrothc syndrom have low plasma albumin concentrations. These condition are associated with an increased fraction of unbound clofibrate and phenytoin, but the steady state concentration of the unbound drug is not altered because of compensatory changes

An adverse response is obvious to the physiciam and the patient, but the prevention of a satisfactory response, which is common, is much less readily discernible. Both may be the result of incorrect prescribing. For example, the "usual" dose of theophylline may not control bronchospasm in the patient who smokes due to induction of theophylline metabolism.

Whereas inhibition of phenytoin metabolism by isoniazid may result in ataxia in patients previously showing no signs of toxicity from the same dose. The latter is most likely to occur in patients who are slow acetylators of isoniazid. The drug interaction becomes clinically significant only in individual with certain genetic determinants.