Morphine

BRAND NAMES

1. MS Contin (extended-release morphine tablets) 

2. Kadian (extended-release capsules) 

3. Avinza (extended-release morphine; discontinued in some regions) 

4. Duramorph (injectable/epidural morphine) 

5. Roxanol (oral liquid morphine) 

6. Oramorph (oral morphine solution, common in some countries)

MECHANISM OF ACTION

Morphine is a strong opioid agonist that produces analgesia by acting on opioid receptors in the central and peripheral nervous system, mainly the μ (mu) receptors.

It works by mimicking endogenous opioids (endorphins) and binding to μ-opioid receptors, which are coupled to Gi protein pathways. This leads to inhibition of adenylate cyclase, reduced cAMP levels, and decreased neuronal excitability.

PHARMACOKINETICS

Absorption

Morphine is incompletely and variably absorbed from the gastrointestinal tract when taken orally due to significant first-pass metabolism in the liver, which reduces its bioavailability to about 20–40%. Because of this, oral doses are usually higher compared to parenteral doses.

Distribution

Morphine has a moderate volume of distribution (Vd), approximately 3–5 L/kg, indicating that it distributes widely into body tissues beyond the vascular compartment.

Metabolism

Morphine is mainly metabolized in the liver by glucuronidation (UGT2B7 enzyme) to form two key metabolites: morphine-3-glucuronide (inactive, may cause neurotoxicity) and morphine-6-glucuronide (active, more potent analgesic). These metabolites are primarily excreted by the kidneys, so liver or kidney impairment can prolong morphine’s effects and increase toxicity risk.

Elimination

Morphine is primarily eliminated through the kidneys in urine, mostly as morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) metabolites, with only a small amount of unchanged drug excreted.

PHARMACODYNAMICS

Morphine is a strong opioid agonist that acts primarily on μ-opioid receptors in the central nervous system and spinal cord. Activation of these G-protein–coupled receptors inhibits adenylate cyclase, reduces cAMP levels, and decreases neuronal excitability. This leads to inhibition of pain signal transmission by reducing presynaptic release of neurotransmitters such as substance P, glutamate, and CGRP, and causing postsynaptic hyperpolarization. The overall effect is potent analgesia, sedation, and euphoria. Morphine also produces dose-dependent respiratory depression, miosis, and reduced gastrointestinal motility.

ADMINISTRATION

Morphine is administered via oral, intravenous (IV), intramuscular (IM), subcutaneous (SC), epidural, or intrathecal routes, depending on clinical need. Oral forms are commonly used for chronic pain, while IV administration is preferred in acute or emergency settings due to rapid onset. It can be given as immediate-release or extended-release formulations. Dosage is individualized based on pain severity, patient response, and tolerance. It should be used cautiously and titrated carefully due to risk of respiratory depression and dependence.

DOSAGE AND STRENGTH

Morphine dosing varies widely depending on route and indication. For adults with acute pain, IV doses may start at 2–5 mg every 2–4 hours as needed, while oral immediate-release doses commonly start at 10–30 mg every 4 hours. For chronic pain, extended-release formulations may be given every 8–12 or 24 hours depending on preparation. Pediatric dosing is weight-based and requires careful monitoring. Dose adjustments are necessary in renal or hepatic impairment due to accumulation of active metabolites.

DRUG INTERACTIONS

Morphine has significant interactions with other central nervous system depressants, including benzodiazepines, alcohol, sedative-hypnotics, and general anesthetics, which can increase the risk of severe respiratory depression and sedation. It may also interact with monoamine oxidase inhibitors (MAOIs), leading to exaggerated opioid effects. Drugs that affect liver enzymes can alter morphine metabolism, and mixed opioid agonist–antagonists may reduce its analgesic effect or precipitate withdrawal.

FOOD INTERACTIONS

Morphine does not have major clinically significant food interactions. However, taking it with food may delay absorption slightly, especially with oral formulations, but does not significantly affect overall bioavailability. Alcohol should be strictly avoided as it can potentiate CNS depression and respiratory suppression.

CONTRAINDICATIONS

Morphine is contraindicated in patients with severe respiratory depression, acute or severe bronchial asthma, and known hypersensitivity to opioids. It should be avoided in cases of paralytic ileus and used with extreme caution in patients with head injury, increased intracranial pressure, or severe liver or kidney impairment due to risk of accumulation and toxicity.

SIDE EFFECTS

• Sedation and drowsiness

• Constipation

• Nausea and vomiting

• Respiratory depression

• Miosis (pinpoint pupils)

• Hypotension

• Dizziness

• Urinary retention

• Risk of tolerance, dependence, and addiction.

OVER DOSAGE

Morphine overdose is a medical emergency that occurs when excessive amounts of the drug are taken, leading to profound central nervous system and respiratory depression. The most dangerous effect is severe respiratory depression, where breathing becomes slow, shallow, or may stop completely, resulting in reduced oxygen levels and possible cyanosis.

TOXICITY

Morphine toxicity is mainly characterized by severe respiratory depression, which is the most dangerous effect and can be life-threatening. Other signs include extreme sedation, pinpoint pupils, hypotension, bradycardia, and coma. In overdose, accumulation of the drug leads to loss of respiratory drive and hypoxia. Treatment includes airway support and administration of naloxone, an opioid antagonist that rapidly reverses opioid effects.