Intrinsic activity of a drug describes:

The intrinsic activity of a drug refers specifically to its ability to activate a receptor upon binding. This concept is essential in pharmacology as it distinguishes how effectively a drug can exert its therapeutic effect once it attaches to its target receptor. A drug with high intrinsic activity can produce a significant biological response even at lower doses, while a drug with low intrinsic activity may bind to the same receptor but elicit a minimal or insufficient response.

In pharmacodynamics, understanding intrinsic activity helps in evaluating how different agonists or antagonists perform. Agonists typically have higher intrinsic activities, meaning they fully activate receptors to produce a response, whereas antagonists usually have low or zero intrinsic activity, as they block receptor activation without promoting any effect.

The other options, while relevant to other pharmacological concepts, do not pertain to intrinsic activity. Metabolism speed relates to pharmacokinetics, drug absorption consistency involves bioavailability, and side effects are concerned with the safety profile and adverse reactions rather than the activation quality of the drug itself.