How specifically a compound targets one receptor type versus related receptors — higher selectivity generally means fewer unintended effects.Receptor selectivity is pharmacologically critical in families with multiple subtypes. Afamelanotide (MC1R-selective) has a different profile than Melanotan II (non-selective MC1R/MC3R/MC4R/MC5R agonist) — the non-selectivity explains why Melanotan II produces both tanning and spontaneous erections simultaneously. Higher selectivity generally means fewer off-target effects; in some cases, intentional multi-receptor engagement is the therapeutic strategy (tirzepatide as GLP-1/GIP dual agonist).