The (S)-(+) enantiomer of mirtazapine is responsible for antagonism of the serotonin 5-HT2A and 5-HT2C receptors,[80] while the (R)-(–) enantiomer is responsible for antagonism of the 5-HT3 receptor.[80] Both enantiomers are involved in antagonism of the H1 and α2-adrenergic receptors,[2][80] although the (S)-(+) enantiomer is the stronger antihistamine.[3]