Agomelatine (Valdoxan) is chemical compound that is structurally closely related to melatonin. Agomelatine is a potent agonist at melatonin receptors and an antagonist at serotonin-2C (5-HT2C) receptors, tested in an animal model of depression (forced swimming test in rodents). It is concluded that the antidepressant-like activity in this model most probably involves a combination of both its melatonin agonist and 5-HT(2C) receptor antagonist properties.

Controlled studies with humans have shown that agomelatine is comparable to paroxetine in treatment of major depression. Agomelatine showed significant benefits over paroxetine due to the complete absence of side effects including the associated sexual side effects that are troublesome with some antidepressants. Because of its actions upon the melatonin receptors, agomelatine shows a marked improvement in sleep quality. However unlike other antidepressants with sedative effects there were no associated instances of daytime drowsiness.

On 27 July 2006 the Committee for Medical Products for Human Use (CHMP) of the European Medicines Agency (EMEA) recommended a refUSAl of the marketing authorisation of Valdoxan/Thymanax. The major concern was that efficacy had not been sufficiently shown. The CHMP had no special concerns about the side effects.