Receptor selectivity describes how preferentially a molecule acts at one target compared with others under specified conditions. It is usually a matter of degree, not an absolute promise that only one biological effect is possible.

Selectivity, affinity and effect are different ideas

Affinity concerns binding. Efficacy concerns the ability of a bound molecule to produce a response. Potency describes how much is needed to produce a defined effect in a particular assay. Selectivity compares activity across targets or systems. Confusing these terms can turn a useful laboratory result into a misleading clinical claim. Boston University: Pharmacology glossary

Selectivity depends on the comparison

A study can only establish selectivity against the targets it examined. A molecule may prefer one receptor at a low concentration but influence additional targets at higher exposure. Tissue distribution also matters: a highly selective receptor interaction can still affect several organs if that receptor is widely present.

An early ipamorelin paper described selective GH-related responses in the models studied. That is a specific pharmacological observation, not evidence that every possible effect or risk was excluded. Raun and colleagues, 1998

What it means for drug development

Selectivity can help researchers separate mechanisms and may improve the balance of effects, but clinical safety requires direct evaluation. Unwanted effects can arise from the intended target as well as from other targets.

When a product is called “highly selective,” ask: selective for what, compared with what, at which concentration, and in which model? Then ask whether the result was connected to a meaningful human outcome.

Selectivity is a useful technical property. It is not a synonym for safe, effective or suitable for personal use.