Morphine (Oramorph / Sevredol / Morcap SR / Morphegesic SR / MST Continus / MXL / Zomorph) is an extremely powerful painkilling drug that is produced by opium poppies. It also inhibits coughing and diarrhoea. It is used medically to relieve the severe pain caused by surgery, cancer, kidney stones and injury.

Morphine acts by stimulating mu-opioid receptors on the surface of nerve cells in the brain and spinal cord. These inhibit pain signals, as well as producing euphoric and sedative effects. Side-effects of morphine include impaired mental performance, drowsiness, blurred vision, reduced appetite and constipation. It can also cause insomnia and nightmares. In high doses, it can inhibit breathing.

Morphine is a highly addictive drug, and is abused as opium. However, morphine is unlikely to cause physical dependence when used medically.

Morphine is given by injection under the skin, into a vein or around the spinal cord (epidurally). Versions of morphine that are taken by mouth are also available, but this is less effective than injected morphine: however, oral morphine, particularly slow-release versions, are more convenient for patients to take at home^[1].

Test tube studies have found that morphine can impair the activity of anti-HIV immune cells, lead to the death of brain cells, increase the ability of HIV to infect white blood cells and stimulate viral replication^{[2][3][4][5][6]}. In addition, studies carried out in monkeys infected with simian and simian-human immunodeficiency virus (SIV / SHIV) have suggested that morphine can increase viral replication and HIV disease progression in the blood and the brain, particularly after repeated exposures to the drug^[7][8]. However, these effects have not been observed in human studies.

Morphine levels may be reduced by protease inhibitors, but this remains to be studied.