What can the variability of strength be between different brands of supposedly identical medications?

Question

I remember reading that medications are within allowable specs if the contain +/- 30% of the labelled strength. I can't find the source of that information in order to check its validity. If that is the case, a 100mg pill would be considered to be within allowable tolerances if it has 70-130mg of the active ingredient. This could mean a 60% change in active ingredient if a patient switches from one brand to another, or if a manufacturer changes its manufacturing process. What is the allowed variability for prescription medications? *I imagine the law/rule could be different for every country, so please specify for which country your answer applies.*

Answer 1

This is a complex subject, but the equivalence of two products (eg., original and a new generic) is established in a controlled clinical trial, typically 10-20 healthy subjects, depending on the drug type. The plasma levels for each drug are measured in each subject given each of the formulations in a randomized fashion. The max blood drug concentration and the AUC are calculated.

The FDA considers two products bioequivalent if the 90% CI of the relative mean C_max, AUC_(0–t) and AUC_(0–∞) of the test (e.g. generic formulation) to reference (e.g. innovator brand formulation) should be within 80% to 125% in the fasting state.

This approach is widely accepted around the world by regulatory health authorities that approve the drug for use in their country,