20.03.4 Effects of Opioids

The reason why opioids are used extensive clinically is that they are potent analgesics. The opioids inhibit the ascending nociceptive information from the spinal cord, so that no pain is felt. The opioids also activate descending pain control circuits to reduce pain.

They reason why opioids are abused extensively is that they cause mood alteration. Opioids, especially heroin, cause a feeling of euphoria and tranquillity.

The diagnosis of opioid abuse is helped by the fact that they cause miosis. Miosis with the opioids is due to constriction of the pupil by an excitatory action on the parasympathetic nerve innervating the pupil that leads to pinpoint pupils. This is termed pathognomonic, which means it is a unique pathological characteristic of opioid use. Some tolerance to the pinpoint pupils does develop, but this is not as great as other tolerances to opioids, and opioid abusers continue to have constricted pupils.

Opioids in high doses can cause nausea and vomiting. This nausea and vomiting is caused by direct stimulation of the chemoreceptor trigger zone, which is the centre in the brain that controls nausea and vomiting.

Opioids in high doses have effects on respiration. The opioids depress respiration by a direct effect on the brainstem respiratory centres. Respiratory depression is the main cause of death with the opioids. For instance, subjects that abuse the opioids become tolerant, and use high doses. When the supply of heroin dries up, this tolerance reverses. If the addict then takes the high dose of heroin that they were using previously, there will be excessive respiratory depression and, possibly, death.

Opioids depress cough. Opioids have a direct effect on the cough centre in the medulla, which leads to a depression of cough.

Opioids also have effects where they cause peripheral vasodilation and inhibition of the baroreceptor reflex. These effects lead to a decrease in blood pressure.

Opioids have effects on the gastrointestinal tract. They inhibit secretions, and they also decrease transit time, which leads to constipation. This is a major problem with both the clinical use and abuse of opioids. Tolerance does not develop to this effect of the opioids.

Opioids also have effects on the urinary bladder. They inhibit the urinary voiding reflex, which leads to urinary retention.