Although still early, the findings open an exciting line of enquiry in a field that has long searched for credible ways to prevent psychosis before it takes hold.

Schizophrenia remains one of the most debilitating psychiatric conditions, affecting not only a person's mental state but also their physical health, education, relationships, and long-term prospects. Doctors have long recognised that many individuals who eventually develop the illness first appear in adolescent mental health services years before symptoms fully emerge.

The new work, led by Professor Ian Kelleher at the University of Edinburgh alongside researchers in Finland and Ireland, analysed health records from more than 56,000 young people in Finland. All had received antibiotics during adolescence while under the care of psychiatric services. Using an "emulated target trial" model - a method that aims to recreate the conditions of a randomised controlled trial using real-world data - the researchers compared those prescribed doxycycline with those given other antibiotics. This approach helps limit the biases that typically hamper observational studies.



The difference between the two groups proved striking. During the ten-year period, the estimated risk of developing schizophrenia stood at around 2.1 per cent for those who took other antibiotics. For individuals who had taken doxycycline, the figure fell to roughly 1.4 per cent - a relative reduction of about 33 per cent. The protective effect appeared consistent across different doses, suggesting that even limited exposure might confer some benefit.

One possible explanation lies in doxycycline's known anti-inflammatory properties and its ability to cross into the brain. Laboratory studies have shown that the drug may help regulate synaptic pruning - the developmental process through which the brain trims unnecessary neural connections. Excessive pruning has been strongly linked to the later onset of psychotic disorders.



Experts not involved in the research have welcomed the findings but urged caution. Some note that the absolute risk reduction remains modest, while others question whether unmeasured differences between the treatment groups could explain the results. Even Kelleher stresses that the study cannot prove causality and that only a randomised clinical trial could offer definitive answers - although conducting such a trial would take many years.

Still, the study presents a rare moment of optimism and hope in an area where progress is slow and the stakes are high. If confirmed, the discovery could mark an important step toward protecting vulnerable young people long before illness takes hold.