In evaluating claims about benefits from an intervention, it is important to understand the difference between absolute risk and relative risk.

Absolute Risk is your actual chance of developing a disease or other measurable situation.  For example, in a clinical trial of a cancer drug, 2 in 100 (or 2%) people taking the drug see tumor growth.  4 in 100 (or 4%) in the control group see tumor growth.   The absolute risk of the patients in the intervention group is 2%; the absolute risk of the patients in the control group is 4%.

Relative Risk compares the risk in two groups. Researchers and pharmaceutical companies often express gains in relative terms because it sounds so good. Using the above example, 4% of people in the control group had tumor growth, but only 2% of those taking the drug saw tumor growth, which means the drug lowered their risk of tumor growth by 50%. 50% sounds impressive! But that’s relative risk. When you think about the absolute risk and ask yourself “50% of what?”, you see that it is 50% of 4%, which is only 2%. These numbers are essential to consider when looking at possible adverse effects that most drugs carry with them. If you were told that you lowered your risk by 50% if you take the more expensive newer drug, you would jump at it, but when you learn that it is 50% of 4% which is only a 2% chance of reduction in tumor growth, you might wonder if the expense and side effects are worth this 2% chance of improvement.

Thinking critically is important when evaluating any medical intervention. Know what you might gain and what you might lose before you decide which drugs you want to take and ask yourself does the benefit outweigh the risk?


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