Therapeutic Nonequivalence of Oxytetracycline Capsules

To the Editor:—  For some time, it has been evident that factors other than those customarily examined enter into the therapeutic usefulness of many drugs. Varley¹ has shown that two chemically equivalent preparations of tolbutamide can differ very significantly in availability to the patient and efficacy of the drug, although both meet official specifications. There are other reports^2,3 on the effect of pharmaceutical factors on drug availability, and Friend⁴ points to the necessity of "making sure by measuring patients' blood levels or by adequate therapeutic testing that a drug preparation varying in any way from one performing satisfactorily is also capable of doing what is claimed for it."We have conducted a series of serum level studies on 16 lots of oxytetracycline capsules, labelled 250 mg potency, and distributed by 13 different suppliers. Each of these lots was certified by the Food and Drug Administration and our