Objective: When advising patients about possible initiation of primary prevention treatment, clinicians currently do not have information on expected impact on lifespan, nor how much this increment differs between individuals. 

Methods: First, UK cardiovascular and non-cardiovascular mortality data was used to calculate the mean lifespan gain from an intervention (such as a statin) that reduces cardiovascular mortality by 30%. Second, a new method was developed to calculate the probability distribution of lifespan gain. Third, we performed a survey in 3 UK cities on 11 days between May-June 2014 involving 396 participants (mean age 40 years, 55% male) to assess how individuals evaluate potential benefit from primary prevention therapies.

Results:
Amongst numerous identical patients the lifespan gain, from an intervention that reduces cardiovascular mortality by 30%, is concentrated within an unpredictable minority. For example, 50-year-old males with national-average cardiovascular risk have mean lifespan gain of 7 months. However, 93% of these identical individuals gain no lifespan, while the remaining 7% gain a mean of 99 months.
Many survey respondents preferred a chance of large lifespan gain to the identical life-expectancy gain given as certainty. Indeed, 33% preferred a 2% probability of 10 years to 5-fold more gain, expressed as certainty of 1 year.

Conclusions:
People who gain lifespan from preventative therapy gain far more than the average for their risk stratum, even if perfectly defined. This may be important in patient decision-making. Looking beyond mortality reduction alone from preventative therapy, the benefits are likely to be even larger.