Side effects are the unwanted effects of a medicine. They are also referred to as adverse effects, adverse events and adverse drug reactions (ADRs).

Your doctor will have prescribed your medicine because of the potential benefits it can give you. You may not experience any side effects but all medicines can cause some side effects in some people. In general, you are more likely to experience side effects when you have just started taking a new medicine for the first time, if your dose has recently been increased or if you have started taking an additional medicine.

In many cases side effects are mild and do not mean that you have to stop taking the medicine. For example, mild stomach upset is commonly experienced with antibiotics. Sometimes, the side effects may be more of a problem and so may require a change to your treatment.

With some medicines, all patients will experience side effects. For example, some types of cancer treatments will cause hair loss and diarrhoea. This is because the anti-cancer action of the medicine also affects other healthy cells in the body (such as hair follicles). If you are taking this type of medicine, the side effects will be closely monitored by your doctor.

In many countries, all medicines, whether purchased over the counter or prescribed by your doctor, are supplied with an information leaflet, referred to as the Patient Information Leaflet or PIL. You should read this carefully before starting any treatment. It will tell you what the possible side effects are and will provide you with useful information and any advice you will need about taking your medicine. Read the leaflet again at regular intervals if you continue to take the medicine, as some information may be updated from time to time.

There are usually several different medicines that can be used to treat your condition. If one does not suit you, ask your doctor if another one might be better. New alternatives do become available and often new medicines are designed to avoid or reduce troublesome side effects seen with older medicines. New medicines undergo very extensive safety testing; however, very rare side effects may not be recognised until these medicines have been widely used for a number of years.