What are side effects?

Side effects are unwanted symptoms caused by medical treatment. They’re also called "adverse effects" or "adverse reactions". All medicines can cause side effects, particularly if you don’t use them as advised. This includes prescription medicines, medicines you can buy over the counter, and herbal remedies and supplements.

Side effects can range from mild, such as drowsiness or feeling sick (nausea), to severe, such as life-threatening conditions, although these are rare. The risk of getting side effects varies from person to person.

You should check the leaflet that is provided with your medication to see if certain side effects could make it unsafe for you to drive or operate machinery.

When to get medical advice

If you think that you or someone you are with may be having a serious allergic reaction to a medicine, phone 999 and ask for immediate medical help. Contact your GP or pharmacist immediately if:

  • you think you have a side effect that is listed as severe in your medicine's patient information leaflet (PIL)
  • you have a side effect you think is serious

You don’t need to see your GP with mild side effects, such as nausea, if you feel you can manage these on your own. Your pharmacist should also be able to tell you if the side effects need further investigation by your GP.

Reporting side effects

You should report side effects from a medicine through the Yellow Card Scheme. There is also now a free app, available for both iOS and Android devices, which allows you to report side effects via your phone or tablet.

For more information, go to How do I report side effects from a medicine?

What side effects can my medicine have?

The PIL supplied with your medicine will list its known side effects. If you no longer have your medicine’s PIL, you can find a copy on the electronic Medicines Compendium (eMC).

The PIL will show whether each side effect is:

  • Very common: more than 1 in 10 people are affected 
  • Common: between 1 in 10 and 1 in 100 people are affected
  • Uncommon: between 1 in 100 and 1 in 1,000 people are affected
  • Rare: between 1 in 1,000 and 1 in 10,000 people are affected
  • Very rare: fewer than 1 in 10,000 people are affected 

Further information:

Page last reviewed: 26/11/2015

Next review due: 26/11/2017