The unfortunate story of the matrix metalloproteinase inhibitor PG116800, which had no effect on the osteoarthritic process but had unexpected side effects, highlights the following. First, reality does not always match the theory. Second, cell biology data must be interpreted within the context of a specific environment. Third, the specificity of an enzyme inhibitor is always relative. Finally, a critical evaluation of the benefit/risk ratio of a drug must be carefully conducted and checked before and after launch. Well designed post-marketing surveillance is mandatory.