Messages that are too sure

Don’t assume you can be sure about the effects of a health action.

It is rarely, if ever, possible to be 100% sure (certain) about how effective a health action is or to know exactly what will happen if we do something for our health.


Knowing exactly what will happen following a health action is almost never possible. This is even clearer when it comes to actions aimed at preventing something in the future. Also, our level of certainty about what will happen can change as new information becomes available. This is especially true for new problems and health actions, such as actions to prevent or treat Covid-19 during the pandemic. Fair comparisons can help us assess the level of certainty of both helpful and harmful effects of health actions.


At the start of the Covid-19 pandemic, little was known about the effects of measures to control the virus. However, in less than a year, over 2000 fair comparisons were started. Dexamethasone – an inexpensive and widely used medicine – was shown to reduce deaths among patients with severe Covid-19 disease. At the same time, another inexpensive and widely-used medicine – hydroxychloroquine – was promoted with great confidence, despite a lack of evidence. Fair comparisons showed hydroxychloroquine was ineffective and had harmful effects. Meanwhile, there have been very few fair comparisons of measures to reduce the spread of Covid-19 (such as lockdowns and closing schools), and the effects of those measures remain very uncertain.

Remember: It is better to recognise that there is nearly always uncertainty about the effects of things we do for our health (health actions) than to make a decision that might be harmful.

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