Scientific evidence to support the use of antidepressants to treat other medical problems is "lacking", a study says.
Antidepressants are sometimes used in a bid to combat other conditions including pain or insomnia.
But a new study found that among all "off-label" antidepressant prescriptions, only one in six prescriptions was supported by strong scientific evidence.
The authors of the new paper, published in The BMJ, said antidepressant use had increased "substantially" in the UK.
"The number of antidepressants dispensed in England increased by 3.9 million (6.8 per cent) between 2014 and 2015 - more than any other therapeutic class of prescription drugs," they wrote.
"One suspected factor underlying the widespread use of antidepressants is an expanding array of indications for these drugs, many of which are unapproved (off-label) for certain antidepressants."
To examine off-label use of the drugs, they looked at all antidepressant prescriptions written by a GPs at two major centres in Quebec, Canada.
During the study period, 106,850 antidepressant prescriptions were written by 174 physicians for 20,920 adults.
Overall, 29.3 per cent of these prescriptions were written for an off-label reason.
Just more than a quarter of such prescriptions were for trazodone used to treat insomnia.
The researchers looked at the evidence behind the off-label prescriptions and concluded that for only 15.9 per cent of all such prescriptions, the drug had "strong scientific evidence" for the reason it was given out.
Almost two in five (39.6 per cent) off-label prescriptions did not have strong evidence themselves but another similar antidepressant was backed by the evidence.
And for remaining 44.6 per cent off-label prescriptions, neither the prescribed drug nor any other drugs in the class had strong evidence for the condition it was prescribed for, they concluded.
"Most off-label antidepressant prescriptions lack strong scientific evidence, but another evidence based antidepressant from the same class could often be considered as an alternative," the authors wrote.
"There is an important need to produce more evidence evaluating the clinical outcomes associated with off-label antidepressant use."
