People with depression could find their drugs are less effective if they also take common anti-inflammatory medicine such as aspirin or ibuprofen, researchers have said.
A team from Rockefeller University in the US found anti-inflammatory drugs can affect SSRI antidepressants, which they said could explain why some patients do not seem to be helped by them.
They said depressed patients may be able to avoid the problem by not taking anti-inflammatory drugs.
The research, published in Proceedings of the National Academy of Sciences, looked at the effects of the two medications in mice and then in human patients.
Two groups of mice were given SSRIs and one group also had an anti-inflammatory. The researchers compared their behaviour as they performed tasks which would be affected by the antidepressants.
The group given SSRIs and anti-inflammatories had inhibited responses, the study found.
Next, the team looked at human patients with depression and found those taking both SSRIs and anti-inflammatory medicines had a significantly lower chance of seeing their depression ease compared with patients only taking SSRIs.
Just 40% of people taking both types of drugs found the antidepressants worked, while 54% of those only taking SSRIs found they helped, the researchers found.