Cheap and widely used drugs for diabetes and heart health have potential for treating severe mental illness, a study hints.
It showed the number of times patients needed hospital treatment fell by up to a fifth when they took the drugs.
The researchers at University College London say their findings have "enormous potential".
But they, and independent experts, say the results now need to be tested in clinical trials.
The starting point for the researchers was a list of currently prescribed medications that science predicts could also help patients with severe mental health disorders.
The team focused on:
- anti-cholesterol drugs called statins - which may calm inflammation linked to mental health problems or help the body absorb anti-psychotic medications
- blood pressure drugs - which may alter the calcium signalling in the brain that has been linked to bipolar disorder and schizophrenia
- type 2 diabetes drug metformin - which may alter mood
But rather than test them in trials, the scientists went looking for evidence in the real world.
They analysed life-long medical records of 142,691 people in Sweden who had schizophrenia, bipolar disorder or other severe mental illnesses.
They then compared the number of times each was admitted to a psychiatric hospital clinic when they were taking those medications and when they were not.
Dr Joseph Hayes, one of the researchers at UCL, said: "The paper suggests a 10-20% reduction in the number of episodes when on the medications rather than off."
The results, published in the journal JAMA Psychiatry, also showed a reduction in self-harm.
"It's incredibly exciting," Dr Hayes said.
"It's got enormous potential and I'm pleased with the way it has turned out.
"But this is really just a starting point."
He wants the drugs to now be tested in large clinical trials, which should give a final answer.
In the meantime, Dr Hayes says people should not go out and try to get the drugs themselves.
But, he says, there are many patients who should be on these drugs for their physical health who are not getting them.
"The thing to do would be to see your GP about full physical health review," Dr Hayes said.
"There's a huge number of people that may benefit from a statin for their heart health and there's a potential knock-on for their mental health, similarly with metformin."
Dr James MacCabe, from the Institute of Psychiatry at King's College London, said: "These findings are very compelling.
"The findings strongly suggest a potential role for repurposing these drugs to improve mental health outcomes."
But there is one nagging doubt, even from the researchers, around the way the study was designed.
A lot of studies compare one group of patients taking a drug with another group not taking it.
This one compared patients at different stages of their life when they were either on the drug or not.
The approach has many advantages but it could mean that when people are in a good place mentally and less likely to be admitted to hospital, they are also more likely to look after themselves and take other medications.
In other words, statins and other drugs could just be a red herring.
This is why Prof Naveed Sattar, from University of Glasgow, remains sceptical and says: "I would be strongly cautious with these findings and would only change my mind if effects are proven to be robust in a randomised trial."
The research group took steps to counter this effect but agree clinical trials are the next step.
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