According to ScienceDirect, extemporaneous compounding is the science of constructing individual chemical components into a drug that is usable by an individual patient. This is generally done by a doctor whose patient needs a product that is not readily available from commercial drug manufacturers.
Extemporaneous compounding is one of the riskiest preparation actions in a pharmacy. According to Keele University, this is mainly due to a large number of reported errors that have been linked to extemporaneously compounded medicines, which ended in serious harm to the patients. Only a small number of extemporaneous compounds are supported by evidence that shows the equivalent results achieved with commercially manufactured drugs.