Principal Uses Of Morphine And Its Analogues

• Relief of moderate to severe acute pain (or chronic pain often in terminal illness)

• Brief relief of anxiety in serious and frightening disease accompanied by pain, e.g. trauma

• Relief of dyspnoea in acute left ventricular failure, and in terminal cancer

• Premedication for surgery

• Symptomatic control of acute nonserious diarrhoea, e.g. travellers' diarrhoea (codeine)

• Suppression of cough (codeine)

• Production of euphoria as well as pain relief in the dying.

Opioid-induced nausea, vomiting and dysphoria may interfere with any of the desired effects.

Dose. There is much individual variation: given s.c. or i.m. morphine 10 mg is usually adequate; with 15 mg unwanted effects increase more than does analgesia; i.v. give (slowly) one-quarter to one-half of the i.m. dose. For oral dosage see Palliative care, page 329. Continuous pain suppression can be achieved by morphine orally and s.c. 4-hourly.

28 Metabolites of morphine appear to underlie the curious phenomenon of allodynia, when a normally painless stimulus is experienced as painful, hyperalgesia, which is the experience of unusually heightened pain from a known painful stimulus, and myoclonia. These have been observed in some patients after large and prolonged doses of morphine. The explanation may involve morphine-3-glucuronide which antagonises the analgesic effect of morphine and morphine-6-glucoronide.

Morphine and disease. When intense peripheral vasoconstriction accompanies, e.g. trauma, morphine administered s.c. or i.m. may appear to be ineffective because it fails quickly to enter the systemic circulation; repeating the dose before the first has been absorbed may lead to poisoning when the vasoconstriction passes off. In such circumstances morphine should be given slowly i.v. (2.5 mg every 2-3 min). If the blood volume is low, morphine may cause serious hypotension.

In hepatic failure small doses can cause coma (see p. 656), and it may be dangerous in hypothyroidism (slow metabolism). In an acute asthmatic attack, morphine is dangerous.

Adverse effects (type A) have been discussed. Dependence and overdose are treated below. Opioid use in obstetrics requires special care (p. 362).

Interactions. Morphine (also pethidine and possibly other opioids) is potentiated by monoamine oxidase inhibitors. Any central nervous system depressant (including alcohol) will have additive effects. Patients recently exposed to neuromuscular blocking agents (unless this is adequately reversed, e.g. by neostigmine) are particularly at risk from the respiratory depressant effects of morphine. The effect of diuretic drugs may be reduced by release of antidiuretic hormone by morphine. Useful interactions include the potientating effect on pain relief of tricyclic antidepressants and of dexamfetamine.

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