Oxytocin

Oxytocin is a cyclic nonapeptide with a single disulfide bridge and an amidated C-terminus, synthesised in magnocellular neurons of the paraventricular and supraoptic nuclei and released from the posterior pituitary. It differs from vasopressin at only two of nine positions, which is why the two cross-react at each other's receptors when concentrations are high. Its receptor is Gq-coupled, raising intracellular calcium in uterine myometrium and in myoepithelial cells around the mammary alveoli.

As an intravenous drug it is long established: synthetic oxytocin is approved for labour induction and augmentation and for postpartum haemorrhage, and carries a boxed warning against elective induction. Its plasma half-life is only a few minutes, so it is given as a titrated infusion. Parallel to it runs a large literature on intranasal oxytocin for social cognition, whose most informative result is negative - a large multi-site trial in children with autism, published in 2021, found no benefit over placebo on social withdrawal.

The gap between those two literatures is a delivery problem as much as a pharmacological one. Peripheral oxytocin crosses the blood-brain barrier poorly, and central concentrations after nasal dosing are inferred far more often than measured, so a behavioural finding may reflect peripheral autonomic effects rather than central receptor occupancy. Early positive results came from small crossover studies with many outcome measures, the design that most reliably generates unreplicable effects.

The claim to distrust is oxytocin as a bonding or trust hormone. That framing came from short laboratory tasks, replicated poorly, and does not survive the negative clinical trials. Marketing for nasal products also elides route: the approved evidence is intravenous and obstetric, where the target is the uterus and the dose is titrated against contractions.