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Side effects (adverse drug reactions)

Any medicine, vaccine, herbal or complementary remedy can cause an unwanted side effect, commonly referred to as an adverse drug reaction (ADR) by healthcare professionals.

Many side effects are linked to the way the medicine works to treat a patient. For instance, medicines known as 'beta-blockers' treat high blood pressure by acting on particular sites in the muscles of blood vessels, causing the muscles to relax, and reducing blood pressure.  This medicine however also acts on the same type of sites in the heart, which may cause an undesirable effect in some patients where the heart beat is slowed down.  

Some side effects are not clearly linked to how the medicine works in the body and so are more unpredictable.  As individuals, different patients may respond in different ways to the same medicine and so it is very difficult to predict whether a patient will experience any side effects. Sometimes side effects can be delayed, happen after long term use or through interaction with food(s) or other medicine(s), on stopping a medicine, or as a result of a medication error, misuse or abuse.

Monitoring side effects after a medicines is licensed

Before a medicine is licensed, it must meet high standards for safety, quality and effectiveness, and medicines are tested in a series of clinical trials.  Such trials are carried out in a relatively small number of patients - on average 1,500 for a new medicine.  In addition, clinical trials are conducted under very strict conditions.  In everyday life, medicines will not be used under trial conditions and may be used by millions of patients across a range of age groups, who may also be taking other medicines and who have varied lifestyles.

Although clinical trials are generally able to identify the more common and predictable side effects of medicines, rarer side effects may only be highlighted once the medicine is used by a far greater number of patients under the conditions of every day use.  In addition, some side effects may not be discovered until many people have used the medicine over a period of time.  This is why it is important to monitor and report information about suspected side effects.

Reporting suspected side effects acts as an early warning system to help the MHRA monitor the safety of medicines through effective regulation. You can about recognised side effects in the patient information leaflet supplied with a medicine.

Asset 3 infographic reporting helps

See our guidance on what to report.

More information about the reporting of suspected side effects or adverse drug reactions is also available on the MHRA website:


See our video below showing the benefits of reporting


Help raise awareness about the importance of reporting suspected side effects

See our resources and campaigns pages for more information and tools to help us raise awareness about the importance of reporting side effects.