When drug therapy is involved in treatment, a frequent conversation revolves around the use of generic drugs in place of the brand-name drug. In the United States generics account for about 72% of the market and that is projected to rise to about 90% in the next six or seven years. For most patients, generic substitution is acceptable and cost effective. For particular circumstances switching to a generic or between generics may present problems of efficacy or tolerability.
In the United States the FDA reviews studies submitted by pharmaceutical companies and either authorizes of denies authorization to the company to market their drug for a particular purpose. In the case of generic authorization, the efficacy and safety of the molecule is a given and so the FDA only requires bioequivalence studies which must demonstrate that the generic product delivers 90% to 125% of the name brand drug. These studies typically have a small n and require a confidence interval of 90%. To say this another way, various pharmacokinetic parameters such as max concentration, time to max concentration, area under the curve, and absorption time fall within the 90% to 125% seen in the branded drug. The FDA does not compare one generic to another but only to the branded drug.
In my office I routinely advise patients to avoid generic substitutes for Tegretol, Effexor, Parnate, Ritalin, and Lamictal (for the first prescription). I also advise patients to examine the prescription label for the name of the generic manufacturer. It is common to discover that one generic product is less effective than another. In those instances, this should be discussed with the pharmacist.
The ultimate resource for generic drugs is the Orange Book which is maintained and updated daily by the FDA.