The main difference when the term Peak is used describing a rising level of a drug in the blood vs the peak activity of the drug is simple but often confused - although important to understand and remember. Peak drug blood level is defined as the highest quantity a drug reaches in your blood, it is only a pure measurement of quantity - like mcg/dl, mg/dl,ng/L, etc.
Conversely Peak activity is the maximum effect of the function of the drug on the body's cells. This is pertaining to the drug-specific effect of any given drug. For example, hCG peaks in the blood within 12 hours (quantity) but does not reach peak activity (testosterone production) until 36-48 hours after the shot. Every drug is different and multiple factors/variables affect both the peak blood level and peak activity.