Morphine is most commonly used as a means of providing pain relief. However, the side effects of morphine vary somewhat and while some of the adverse effects seen with initial therapy are short-term, others may be long-term, especially when the drug is used over longer durations of time.

Some common side effects associated with morphine use include:

• Gastrointestinal side effects - These include nausea, vomiting, abdominal cramps and constipation. Opioid receptors for morphine are present in the gastrointestinal tract and their activation may lead to a slowing of gut movement, leading to constipation. Long-term use leads to severe constipation. In addition, morphine stimulates the vomiting centre of the brain causing nausea and vomiting. Morphine and codeine cause more nausea than other opioids. There may be drying of the mouth as well.
• Sedation - Named after Morpheus, the Greek god of dreams, morphine is one of the most powerful sedative opioids. People taking morphine are therefore advised to refrain from driving and operating heavy machinery to avoid accidents.
• Skin changes - Morphine may lead to the release of histamine in the skin leading to warmth, flushing and urticaria or allergic eruptions across the skin. Skin may appear cool and clammy and a severe chill called hypothermia may develop.
• Shrunken pupils - Morphine may cause the pupils to constrict and appear pin-pointed in size.
• Respiratory depression - The breathing mechanism can become suppressed in response to low blood oxygen. In healthy individuals, as blood oxygen falls and blood carbon dioxide rises, the drive for breathing increases. However, morphine suppresses this drive in the brain, potentially causing this very dangerous side effect. Respiratory depression is more common with higher doses.
• Initial doses lead to euphoria but at higher doses unpleasant symptoms such as hallucinations, delirium, dizziness and confusion manifest. There may be some amount of headache and memory loss.
• Biliary colic and subsequent severe abdominal pain is common in morphine overdose.
• With high doses, muscle rigidity and abnormal movement of the limbs and muscles called myoclonus may manifest.
• One of the prominent side effects of morphine use is the development of physical or psychological dependence on the drug and withdrawal symptoms when the drug is stopped. Morphine is a highly addictive substance and some of the withdrawal symptoms include pain, loss of sleep, nausea, vomiting, diarrhea, goose bumps, hot and cold flashes and intense craving.
• Development of tolerance - Tolerance describes when a person needs to take a drug in higher doses to achieve the same degree of e.g. euphoria or pain relief as before. Developing a tolerance in this way often means addicts use higher doses of the drug.
• Transmission of diseases such as HIV/AIDS and hepatitis B and C among drug abusers who use needles to inject the drug.