Up to 11 per cent of people taking an opioid—which include codeine and morphine—experience depression for the very first time, and usually after taking it for more than a month, and irrespective of the dose being used.
Researchers at St Louis University have discovered that opioids can improve mood in the short-term, but start causing biological changes, including neurological re-patterning, when they are taken for longer than 30 days. They also lower levels of testosterone.
They looked at three research studies, and discovered that the rate of new cases of depression among participants were very similar, and range between 9 and 11 per cent.
Doctors need to be made aware of the effects and monitor patients carefully, especially if they have taken the drugs for more than a month. They should also seek out alternatives for treating depression.