Only what the 36 mg arm reported, and the denominator is that arm rather than the trial. Adverse-event tables are published per arm, so the 36 mg incidence of injection-site erythema has its own numerator and its own denominator, and pooling it with the other arms produces a figure that describes nobody. Two further deductions before you use it. Subtract the placebo arm — injection-site erythema occurs in people who received nothing, and the difference is the part attributable to the drug. And check whether ATTAIN-1 counted events or counted participants: one participant with six episodes is one row in a participant count and six in an event count, and the two get quoted interchangeably. Nothing here is medical advice.
The trial answers a narrower question than the headline suggests, and the narrowing is where the useful information is.
Placebo arms in this class are not nothing. Lifestyle-intervention placebo arms in the major obesity trials commonly lose two to three per cent of body weight, so an active-arm figure quoted without its comparator overstates the drug effect by roughly that much.
The underlying point is that a composite endpoint is only as informative as its least serious component. Where a cardiovascular composite combines death, infarction and stroke, ask which component moved, because they are not interchangeable outcomes.
Be careful about generalising from a trial population to yourself. The exclusion criteria are usually the most informative page in the supplement.
The short version: check the endpoint, check the comparator, check who was excluded, then look at the number.