In more complex analysis, death and disability are often evaluated in combination as the quality-adjusted life year (QALY); treatment that results in an additional year of life at 100% of normal functioning is credited with 1 QALY; treatment that results [merckmanuals.com]

Thus, if a drug causes a 3% net decrease in mortality, 1/0.03 = 33.3 patients need to be treated to prevent 1 death. The number needed to harm is similar. [msdmanuals.com]