New multisource (generic) pharmaceutical products must be of good quality and at least as safe and efficacious as existing products. The need for interchangeability arises when a patient may change from one brand to another, for example in these circumstances:

X Physicians prescribe by generic name;

X Generic substitution is permitted by national legislation;

X The same brand is not always available, for example in remote areas of the country;

X Patients in hospitals are given whatever brand the hospital has in stock, and sometimes different brands on different occasions;

X Patients receive a different brand after discharge from hospital.

WHO’s guidelines on “Multisource (generic) pharmaceutical products” mentions a number of features which are important to interchangeability (see Annex 3, Part one, section 2), and each of these is examined in Annex 7. The science behind demonstrating interchangeability is still evolving; an example is the current debate in the literature on the relevance of “intestinal permeability” (12) and how to measure it.

By their nature, different brands of modified (sustained, continuous, prolonged, slow) release products are more likely not to be equivalent than are different brands of immediate, conventional release products. Some DRAs take the view that such products should never be considered interchangeable, while others define a series of studies that should be conducted, including in some circumstances comparative clinical trials. For delayed release products, such as enteric-coated tablets, interchangeability is more readily demonstrated.