Opioid analgesics are the agents of choice for many patients. Opioids act by binding with specific receptors in the brain, spinal cord, and peripheral nervous system. Morphine has a duration of action of 3 to 4 hours when given parenterally, is metabolized by the liver, and undergoes renal excretion. It has minimal cardiovascular effects at therapeutic doses but can cause decreased cardiac contractility and histamine release, resulting in vasodilation (arterial and venous), urticaria, and bronchospasm. It also causes decreased gastrointestinal (GI) motility. Meperidine should be avoided due to its many side effects. Lethal interactions can occur when meperidine is given to patients taking monoamine oxidase inhibitors. Fentanyl, a synthetic opioid that causes no histamine release or decrease in cardiac contractility, has a short half-life, which makes it excellent for procedures and is lipid soluble for rapid onset of action. High or repeated doses can cause muscle rigidity. Hydromorphone is very soluble, allowing small volumes to be used in injection. It has a duration of 2 to 4 hours when given parenterally and is also available in oral and rectal forms.

Oral opioids are indicated for moderate pain states. These agents undergo first-pass metabolism, so the doses used are often greater than parenteral doses of the same
drug. They are often prescribed in combination with acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID). Codeine has limited analgesic effect and is not a very potent analgesic. It often causes nausea and sedation but is an excellent antitussive. Hydrocodone, a synthetic analog of codeine, offers increased analgesia with decreased GI side effects. Oxycodone is a potent oral analgesic but causes more euphoria, increasing abuse potential. Oral preparations of morphine and hydromorphone are available. Morphine is available in long-acting preparations that are most commonly used in chronic pain states. Methadone