Abstract Morphine is the main pharmacologically active constituent of opium, the resin derived from the dried juice of the opium poppy, Papaver sominferum. Morphine was first introduced into clinical practice more than 200 years ago. Its effects are mediated by activating opioid receptors, mainly within the central nervous system (CNS). Globally morphine remains the strong opioid of choice for moderate-to-severe pain. Other strong opioids are generally used when morphine is not readily available or the patient has intolerable adverse effects with morphine. The WHO has placed morphine on its Essential Drugs List.