Efficacy, NNT & Safety


- Some people don’t feel comfortable with numbers, but numbers are vital for making informed health decisions.

- Efficacy is measured as the difference between cases in the drug (intervention) group and cases in the placebo (control) group. The size of the trial doesn’t matter. For a drug intended to prevent disease, an efficacy figure of, say, 90% is usually thought to mean that, if 100 people take the drug, 90 people won’t get the disease. It doesn’t mean this. A trial can take place, with virtually no one getting the disease – drug and placebo groups alike – but the efficacy number can still be 90%. Efficacy gives us relative differences.

- The number of cases in the drug group and cases in the placebo group allow us to calculate absolute differences and these are the ones that can far better inform decisions. The number needed to treat (NNT) is an important measure. This tells us how many people need to take a drug for one person to benefit. If everyone who takes the drug (or intervention) benefits, there is a number needed to treat of one. If 1 in 100 person benefits, that’s a NNT of 100. People would be more likely to accept an intervention with an NNT of 1 (they will benefit) vs 100 (they have a 1 in 100 chance of benefiting).

- Safety is the final dimension to factor in. There is no point having an intervention with 100% efficacy and a NNT of 1 (everyone benefits) if everyone is harmed (or even if a high proportion of people are harmed). The number needed to harm (NNH) is also important to make an informed decision.

- There are reporting schemes, calculators and websites to try to help with the information required to make the best health decisions. Unfortunately, not enough people know about these to report information that could help others and to review the information for themselves.

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