primary prevention

Medicine draws a line between two jobs that look similar and are not. Primary prevention is aimed at someone with no diagnosed cardiovascular disease, and its purpose is to stop a first heart attack or stroke from ever happening. Secondary prevention is aimed at someone who has already had one, and its purpose is to stop the next.

The distinction decides who is offered what. NICE, the body that sets treatment guidance for the NHS in England, splits its cholesterol guideline along exactly this line. One section covers assessing cardiovascular risk in people without established cardiovascular disease, another covers lipid-lowering treatment for primary prevention, and a separate section covers secondary prevention, where the guideline recommends offering atorvastatin 80mg to people who already have cardiovascular disease, whatever their cholesterol level.

The distinction also decides how strong the evidence has to be. A patient who has already had a heart attack faces a high enough risk that a drug's benefit is comparatively easy to demonstrate. In primary prevention the underlying risk is lower, the benefit is therefore smaller, and any side effect weighs more heavily against it. That is why primary prevention questions need large trials and years of follow-up before they can be answered.