‘Expiry date’ is mentioned on the labels of some medicines. On what basis is it determined?

Some decades ago there were no chemists’ shops. Instead of chemists druggists used to mix various compounds according to physicians’ prescriptions and sell them either in powder form or liquid form. (Picture on the right). Sometimes physician's assistant used to prepare such mixture in the dispensary itself so he was known as compounder. These types of medicines did not require a cautionary notice of ‘expiry date’ to be written on the label because patients used to consume the medicine within a few days. Over a period of time pharmaceuticals manufacturing companies started manufacturing and selling such compounds in the form of tablets and capsules. Chemists’ shops replaced druggists' stores. Now there was no longer certainty that tablets and mixtures were consumed immediately after production. If medicines remain unused for a long time some changes may take place in the properties of the ingredients due to heat and moisture. This would reduce efficacy of medicines, so it became essential to write 'expiry date’ on the packaging material of medicines. The best method for determination of date of expiry is to store the tablets or capsules for a long period at the normal room temperature but pharmaceutical companies resort to short cuts to avoid such time consuming method. They store four samples of medicine at four different temperatures for a fixed period of time and then analyse the effects on the ingredients of the medicine. They ascertain the extent of loss of efficacy of these samples and determine ‘expiry date’ based on the average loss of efficacy.