A 2015 study has found that larger opioid medication doses increase the incidence of depression in a Veterans Administration study of 355 pain patients. An opioid is a pharmaceutical compound, such as morphine, that produces an analgesic effect in the nervous system.

The study, published in the Journal of the International Association for the Study of Pain, followed patients with low-back pain for two years. The patients were taking varying doses of opioid pain killers, rated by their morphine-equivalent dose. The researchers found that higher doses resulted in a doubling of depression incidences.

According to Dr. James Duke, author of The Green Pharmacy, natural herbal alternatives to painkiller drugs that are free of the side effect include meadowsweet, ginger, willow bark, clove, lavender, eucalyptus, red pepper and rosemary.