neurotransmitter agent miotic Aa agent causing contraction of the pupil of the eye. Because the size of the pupil is under the antagonistic control of the sympathetic and parasympathetic systems, drugs affecting either system can cause miosis. Drugs that mimic or potentiate the parasympathetic input to the circular constrictor muscle and drugs that inhibit sympathetic input to the radial dilator muscle tend to contract the pupils. CHEBI:51068 Miotico chebi_ontology miotics