OPIOID ANALGESICS

Opioid analgesics are drugs derived from opium or synthetic analogs, primarily prepared from the poppy plant (Papaver somniferum). They are the most effective pain relievers available and are commonly used as first-line therapy for the management of:

• Acute severe pain
• Moderate to severe chronic pain associated with cancer, AIDS, or other life-threatening conditions

Classification:

1. Weak opioid analgesics: e.g., codeine.
2. Strong opioid analgesics: e.g., morphine, pethidine, and methadone.

Mechanism of Action:
Opioids relieve pain by mimicking the effects of endogenous opioid peptides (like endorphins), primarily by binding to the mu-opioid receptors. These receptors are found in both the ascending and descending pain pathways in the brain and spinal cord. By binding to these receptors, opioids alter pain perception and can produce euphoria and relaxation, which help alleviate the stress and emotional distress that often accompanies severe pain.

Codeine

Available Preparations:

• Tablets: 30 mg

Indications:

• Mild to moderate pain
• Cough suppression
• Diarrhea

Contraindications:

• Respiratory depression
• Obstructive airway disease
• Hypersensitivity to codeine
• Acute alcoholism
• Risk of paralytic ileus
• Raised intracranial pressure or head injury

Dosage:

• Relief of pain:

1. Adults: 30 mg–60 mg every 4–6 hours, maximum dose of 240 mg/day.
2. Children (1–12 years): 0.5 mg–1 mg/kg every 4–6 hours.

• Diarrhea:

1. Adults: 30 mg 4–6 times daily.

Pharmacokinetics:

• Well absorbed after oral administration.
• Widely distributed in the body, crosses the placenta, and enters breast milk.
• Metabolized in the liver and excreted primarily in urine.

Side Effects:

• Constipation
• Dry mouth
• Facial flushing
• Nausea and vomiting
• Difficulty urinating
• Headache and dizziness
• Sweating

Drug Interactions:

• Alcohol and other CNS depressants increase the sedative effects of codeine.
• Rifampicin and phenytoin may increase the accumulation of codeine.
• Severe cardiovascular depression may occur when used with general anesthetics.

Nursing Considerations:

• Encourage increased fluid and fiber intake to prevent constipation.
• Avoid alcohol during codeine therapy.
• Avoid abrupt discontinuation after prolonged use to prevent withdrawal symptoms.
• Codeine is not recommended for productive cough.
• Administer with food to minimize nausea and GI upset.

Morphine

Type:
Strong, centrally acting opioid analgesic.

Available Preparations:

• Oral solution: 5 mg/5 ml, 10 mg/5 ml, 50 mg/5 ml
• Injection: 10 mg/ml, 15 mg/ml

Indications:

• Severe pain (e.g., post-operative, cancer pain)
• Myocardial infarction
• Premedication before surgery
• Sickle cell crisis
• Acute pulmonary edema
• Chronic pain

Contraindications:

• Acute respiratory depression
• Hypersensitivity to morphine
• Acute alcoholism
• Head injury
• Acute abdominal pain
• Raised intracranial pressure
• Avoid injections in patients with pheochromocytoma

Pharmacokinetics:

• Absorbed variably from the gastrointestinal tract (GIT).
• Widely distributed throughout the body.
• Metabolized mainly in the liver.
• Excreted in urine and bile.

Mechanism of Action:
Morphine binds to opioid receptors in the central nervous system (CNS), altering both the perception of and emotional response to pain. It has both depressing and stimulating effects on the CNS:

Depressing effects:

1. Reduces the brain’s appreciation of pain.
2. Depresses respiration.
3. Depresses the cough reflex.
4. Acts as a mild hypnotic, inducing sleep or drowsiness.
5. Causes euphoria and reduces anxiety.

Stimulating effects:

1. Stimulates the chemoreceptor trigger zone, causing nausea and vomiting in some patients.
2. Causes pupil constriction due to effects on the third cranial nerve.
3. Decreases bowel peristalsis, leading to constipation.

Dosage:

Acute pain (post-operative pain):

1. Oral: 5–20 mg every 4 hours.
2. Subcutaneous (SC) or intramuscular (IM):

• Adults: 10 mg every 4 hours if necessary.
• Children (6–12 years): 5–10 mg every 4 hours.
• Children (1–5 years): 2.5–5 mg every 4 hours.
• Neonates: 150 mcg/kg 4 times a day.

Chronic pain:

1. Adults: 10–15 mg every 4 hours.
2. Children (1–12 years): 200–400 mcg/kg every 4 hours.

Side Effects:

• Nausea and vomiting
• Constipation
• Respiratory depression
• Postural hypotension
• Urinary retention
• Euphoria or hallucinations
• Sweating
• Bradycardia
• Decreased libido
• Dependency

Drug Interactions:

• Increases sedative effects when combined with antidepressants, antipsychotics, or sedating antihistamines.
• May lead to severe cardiovascular depression when used with drugs metabolized by the liver (e.g., phenytoin, rifampicin).

Nursing Considerations:

• Avoid alcohol during morphine therapy.
• Prolonged use may lead to dependence, and abrupt discontinuation should be avoided.
• Caution patients about the potential for low blood pressure and blurred vision.
• Naloxone can be used to treat morphine overdose.
• Watch for urinary retention, especially in patients with prostatic hypertrophy.

Pethidine

Type:
Synthetic opioid, less potent than morphine but equally effective in pain management.

Available Preparations:

• Injection: 50 mg/ml, 100 mg/ml

Routes of Administration:
Intramuscular (IM), intravenous (IV), subcutaneous (SC), or oral.

Indications:

• Pre-operative medication
• Acute analgesia (e.g., post-operative, obstetric)
• Moderate to severe acute pain

Contraindications:

• Hypersensitivity to pethidine
• Acute respiratory depression
• Severe renal or liver disease
• Head injury and raised intracranial pressure

Pharmacokinetics:

• Well absorbed orally, with a bioavailability of 50%.
• Onset of action: 10–15 minutes after oral administration.
• Short duration of action: 2–3 hours.
• Metabolized in the liver; excreted in urine.

Dosage:

Acute pain:

• Adults: 50–150 mg, repeated after 4 hours if necessary.
• Children: 0.5–2 mg/kg every 4 hours.

Obstetric analgesia:

• 50–100 mg, repeated every 1–3 hours, with a maximum dose of 400 mg/day.

Side Effects:

• Nausea and vomiting
• Constipation
• Respiratory depression
• Postural hypotension
• Urinary retention
• Sweating, palpitations
• Dependency, hallucinations
• Bradycardia

Drug Interactions:

• Phenothiazines may cause severe hypotensive episodes and prolonged respiratory depression.
• Alcohol and other CNS depressants potentiate respiratory depression.
• Cimetidine may decrease pethidine elimination, increasing the risk of toxic effects.

Nursing Considerations:

• Prolonged use may lead to physical dependence.
• Use the lowest effective dose, especially in labor.
• Avoid alcohol during therapy.

Opioid/Narcotic Antagonist:

Naloxone

Type:
A drug that reverses the effects of opioid analgesics.

Indications:

• Opioid overdose (e.g., morphine overdose)
• Neonatal asphyxia due to opioid use during labor
• Diagnosis of opioid dependence (it worsens withdrawal symptoms)
• Opioid-induced respiratory depression

Dosage:
0.4 mg intravenously (IV).

Mechanism of Action:
Naloxone competitively blocks the actions of opioid peptides at opioid receptors, reversing their effects.

Pharmacokinetics:

• Well absorbed via IV, with an onset of action in 2–3 minutes.
• Undergoes first-pass metabolism when given orally.
• Metabolized in the liver, excreted in urine.
• Duration of action: 3–4 hours.

Side Effects:
Rare at normal doses, but adverse effects may include:

• Hypertension
• Tachycardia
• Hyperventilation
• Nausea and vomiting

Nursing Considerations:

• Monitor the patient’s response and adjust the dose as needed.