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 they’re not used as prescribed.

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

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 listed as serious 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.

Reporting side effects

You should report side effects from a medicine through the Yellow Card Scheme. 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: 24/03/2014

Next review due: 24/03/2016