Drugs with a short duration of action generally have weaker bonds while long-duration or irreversible drug-receptor interactions may have stronger bonds such as covalent. Ligands bind to precise molecular regions, called recognition sites, on receptor macromolecules. Agonists activate receptors to produce the desired response. Antagonists prevent receptor activation and produce various effects. Receptor antagonists can be reversible or irreversible. Reversible antagonists readily dissociate from their receptor whereas irreversible antagonists form a stable, permanent or nearly permanent chemical bond with their receptor.