To assess whether drug treatment in common practice can prevent disease, we analysed four preventive cardiovascular randomised clinical trials (RCTs), expressing efficacy by 1-year Number Needed to Treat (NNT) in RCT and common practice effectiveness by the Disease Impact Number (DIN) in all subjects at risk and by the Population impact Number (PIN) in the entire population, based on a Swedish population survey. Adjustments were made for non-adherence. Calculations were made of alternative 1-year drug costs and number of years an average general practitioner (GP) would need to work in order to prevent one event using the actual treatment. Secondary prevention of MI by simvastatin (NNT, DIN and adjusted PIN = 37, 93 and 2657; GP work time 2.7 years; drug costs Euro 1020 - 13505), and prevention of stroke by antihypertensive treatment in high-risk subjects (elderly with systolic blood pressure > 160 mm Hg; NNT, DIN and adjusted PIN = 167, 239 and 11950; GP work time 6 years; drug costs Euro 6095 - 51567) appeared medically and economically effective. Primary prevention of MI by pravastatin (NNT, DIN and adjusted PIN 208, 2080 and 24470; GP work time 12.2 years; drug costs Euro 5736 - 117676) or by antihypertensive drug treatment in low-risk subjects (diastolic blood pressure 90-99 mm Hg) (NNT, DIN and adjusted PIN 1667, 3334 and 116982; GP work time 58.5 years; drug costs Euro 60895 - 511718) seemed ineffective and expensive.