Opioids are most commonly used for pain relief and while some of the adverse effects seen with initial therapy are short term, some may last for longer durations, especially when therapy is continued over longer periods.

Some of the adverse effects of opioid use include:

• Nausea and vomiting: The use of opioids stimulates opioid receptors present in the gastrointestinal tract as well as in the vomiting centre of the brain to cause nausea and vomiting. Some opioids such as morphine and codeine cause more nausea than other opioids.
• Drowsiness or sedation: Opioids, and in particular morphine, are known to cause severe sedation and drowsiness. People taking opioids are thus advised to refrain from driving and operating heavy machinery in order to avoid accidents.
• Skin changes: An allergic reaction called urticaria may develop and cause a skin rash characterized by red, itchy, raised bumps. This is caused by the release of histamine in response to opioid use. Individuals may also experience flushing or cooling of the skin. In the case of cooling, skin may appear cool and clammy and an individual may shiver or even develop hypothermia.
• Miosis: This describes the formation of small, constricted pupils, similar to how pupils respond to bright light.
• Constipation: Opioids cause sluggish peristaltic movements in the digestive tract. This causes stasis or loss of movement of the intestinal contents and leads to severe constipation, especially in the case of long-term use.
• Respiratory depression: The breathing mechanism in response to a low blood oxygen level may be suppressed. As blood oxygen falls and blood carbon dioxide rises, there is an increase in drive for respiration. However, opioids adversely affect the ventilatory response to hypoxemia, therefore increasing the risk for hyopventilation. However, this occurs mostly in cases of opioids that are more potent and taken in higher doses
• Psychological effects: Opioids give rise to a sense of euphoria and may also lead to hallucinations, delirium, dizziness and confusion. There may be some amount of memory loss and headache.
• Changes in heart rate: Heart rate may become either rapid or very slow. Some opioid users may also develop postural hypotension or a severe fall in blood pressure on standing up from a sitting or lying position.
• Spasms: Some people may develop spasms of the ureter and urinary retention or biliary colic and spasms of the biliary tree.
• Myoclonus: This describes muscle rigidity and abnormal movement of the limbs and muscles. This can occur with the use of high doses.
• Dependence and likelihood of abuse: Long term opioid use may cause dependence on the drugs, leading to withdrawal syndrome if they are abruptly discontinued.