{"id":3024,"date":"2017-03-29T14:19:28","date_gmt":"2017-03-29T13:19:28","guid":{"rendered":"https:\/\/bestpractice.bmj.com\/?page_id=3024"},"modified":"2026-01-20T17:22:01","modified_gmt":"2026-01-20T16:22:01","slug":"understanding-risk","status":"publish","type":"page","link":"https:\/\/bestpractice.bmj.com\/info\/us\/evidence\/practise-ebm\/understanding-risk\/","title":{"rendered":"Understanding risk"},"content":{"rendered":"

What is a risk?<\/h3>
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You may find the following information useful to share with patients who would like to understand more about the numbers involved in interpreting the benefits and harms of treatments. You may also find some ideas about how to discuss risk with your patients.<\/p>
\r\nA risk is the chance that something will happen. For example, if you smoke a packet of cigarettes a day for 30 years, research suggests you have a 10 percent risk of dying from lung cancer.[1]
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\r\nWhen treating a patient, doctors use numbers from research studies to tell them which treatments are likely to work for that person. These numbers also tell them about the risk of side effects. If you’re a man and your surgeon says you need your prostate removed, there’s a risk you’ll have erection problems afterwards. Your surgeon may think the risk is too low to worry about. But you may think any chance is too high. This is why you need to understand what risk means \u2013 so you can take part in treatment decisions.
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\r\nImagine you’re tossing a coin. It has two sides: heads and tails. So, you have a 1 in 2 chance that it’ll come up heads, and a 1 in 2 chance that it’ll come up tails. It’s the same chance every time, however many times you flip it.
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\r\nA 1 in 2 chance can also be written as a 50 percent chance. ‘Percent’ just means ‘out of a hundred’, so 50 percent looks like this:
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\r\nHere’s a medical example. Imagine your doctor says: “There is a 50 percent chance you will be cured by this drug.” If 100 people like you were treated, the chances are that 50 of them (the red dots above) would not be cured, while 50 (the white dots) would recover. Here are two more examples:
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\r\nThe thing to remember is that, in both cases, the white dots show your chance of being fine.
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\r\nIf you see numbers like 0.8 percent, this means the risk is less than 1 in 100. The more zeros there are after the decimal point, the lower the chances. For example:
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