What is a risk?

A risk is the chance that something (usually bad) will happen to you because of something else. For example, if you smoke a pack of cigarettes a day you have a 1 in 10 risk of dying from lung cancer.¹

Doctors use numbers from research studies to tell them which treatment is likely to work for you. These numbers also tell them about the risk of side effects. If you're a man and your surgeon says you need to have your prostate removed, there's a risk you'll have problems getting an erection. Your surgeon may think the risk is too low to worry about. But you may think that any chance of this happening is too high. This is why you need to understand what risk means: so you can take part in making decisions about your treatment.

In the next section, we'll explain ways that you can use risk to understand the effects of treatments.

Now read: How do I use risk to choose treatments?