Your chance of getting side effects
| Description of side effects | Meaning |
| Very rare | Fewer than 1 in 10,000 people taking the medicine are affected |
| Rare | Between 1 in 1,000 and 1 in 10,000 people taking the medicine are affected |
| Uncommon | Between 1 in 100 and 1 in 1,000 people taking the medicine are affected |
| Common | Between 1 in 10 and 1 in 100 people taking the medicine are affected |
| Very common | More than 1 in 10 people taking the medicine are affected |
You are more likely to develop a side effect if it is described as occurring in 1 in every 10 people taking the medicine, than if it is described as occurring in 1 in every 10,000 people taking the medicine.
As an example, ‘common’ would mean that if everyone living in a street was taking the same medicine, then 1 person might develop the side effect. For side effects described as ‘very rare’, only 1 person out of a group the size of a small town, who are all taking the same medicine, would be expected to get the side effect.
Your age, general health, diet and the use of alcohol and other medicines can influence the likelihood of side effects occurring.
Benefit:Risk ratio
Doctors and pharmacists sometimes talk about a medicine’s ‘benefit:risk ratio’. What they mean is how likely it is that you will gain benefit from a medicine compared with how likely it is that you will develop side effects.
In general, medicines used to treat more serious health conditions can cause more serious side effects. Your doctor will weigh up the benefits of treatment against the risks and severity of any possible side effects. He/she will discuss these with you and will also consider the possible outcomes of not treating your condition, for example if no other treatment is available. This will help you and your doctor decide on the best course of treatment for you. A medicine will only be prescribed for you if the expected benefits are greater than the possible side effects.
For example, only a very low risk of side effects would be acceptable from the treatment of minor conditions, such as the common cold. On the other hand, for more serious or life-threatening conditions, such as cancer, a higher risk of side effects is acceptable owing to the potential risks involved if the disease is not treated.
