Oxytocin

What it is

Structurally, oxytocin is a small ring-shaped peptide of nine amino acids, a shape chemists call cyclic. The body's own version is released from the brain into the bloodstream and also acts as a signal between nerve cells in the brain itself.

Its clearest, best-established job is on uterine muscle: it binds a receptor on those muscle cells and triggers the calcium signals that cause the uterus to contract. That mechanism is why a synthetic version of the hormone is used to start or strengthen labor and to control bleeding after childbirth. A separate strand of research looks at oxytocin's action in brain circuits tied to social behavior, though the receptor targets involved there are less precisely mapped than the uterine mechanism.

How it is studied

The obstetric use of oxytocin rests on decades of clinical use and drug-label data rather than a single trial. Its research into social and mood-related effects has been studied mainly through small to mid-sized randomized trials in adults, generally using a nasal spray form of the hormone and comparing it against a placebo, a dummy spray with no active hormone.

What studies report

• In an 8-week trial in adults with obesity, oxytocin given four times daily did not reduce body weight or improve body composition compared with placebo. [1]
• In that same 8-week trial, people taking oxytocin ate less at a test meal than people taking placebo, and no serious side effects were reported in either group. [1]
• In a 12-week trial in people with schizophrenia, oxytocin given twice daily did not outperform placebo on its main measures of social thinking, but showed a modest advantage on one part of social functioning and a within-group drop in negative symptoms such as low motivation and flat emotion. [2]
• In a large 24-week randomized trial in children and adolescents with autism spectrum disorder, intranasal oxytocin did not significantly improve social withdrawal or other measures of social and cognitive functioning compared with placebo, and side effects were similar between the oxytocin and placebo groups. [3]

What is not known

The clinical trial record for oxytocin's effects on social behavior, mood, and autism-related traits is mixed rather than consistently positive, and several well-run trials, including the large pediatric autism trial and the obesity trial summarized above, did not find the main benefit researchers were testing for. It remains unclear which patient groups, if any, are most likely to respond to the hormone outside its established obstetric use, and it is not yet known whether a smaller effect, such as reduced eating at a single test meal, translates into any change that matters over months or years.

Reported adverse findings

• Trials reported no serious adverse events with repeated nasal use over 8 to 24 weeks, though the studies did not find the behavioral benefit being tested for. [1][2][3]

Regulatory status

A synthetic form of oxytocin (marketed under names including Pitocin and Syntocinon) is FDA-approved for starting or strengthening labor contractions and for controlling bleeding after childbirth, given by injection or intravenous infusion in a hospital setting, where drug labeling describes a risk of overstimulating the uterus and, at high infusion amounts, water intoxication. The intranasal form studied for social and behavioral research is not FDA-approved for those uses and remains investigational.