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Common Opioid Analgesics

Meperidine (Pethidine)

Common name is pethidine. It is a synthetic drug.

Is less potent than morphine, binds to kappa receptor.

Given parentally, hepatic oxidation converts it into active metabolite, normepridine, which may cause seizures.

Less spasmogenic has no cough suppressive ability.

Meperidine is used in obstetrics. Unlike morphine, meperidine produces no respiratory depression in the fetus.

It does not release histamine, thus hypotension is less.

Side effects                                                                                 

Related to morphine sedation.

  1. Antimuscarinic/vagolytic actions
  2. Nausea and vomiting
  3. Atropine like effects (stimulant action initially) less, less miosis and constipation. Causes tachycardia.
  4. Mydriasis occurs in over dosage
  5. Excitation, hallucinations, tremors, delirium, seizures.
  6. Dryness of mouth, myoclonus because of excitation, unpleasant dreams
Contraindications

Morphine is contraindicated in pregnancy but pethidine can be given.

Contraindicated in patients of tachycardia

  Morphine Pethidine
Source Natural Synthetic
Chemistry Phenanthrene Phenyl piperidine
MAO µ agonist µ agonist and anticholinergic
Potency 10 times potent Less potent
Pharmacological actions Powerful analgesicMiosis

Cough suppression

Less powerfulMydriasis

No cough suppression

Respiratory effects Severe depressionbronchoconstriction LessBrochodilation
Anticholinergic actions No anticholinergic effectSpasmogenic

Urinary retention

Antidiarrheal/constipation

Possesses effectsSpasmolytic

None

None

Uses Severe visceral pain, MIAntidiarrheal Moderate visceral pain, not MIRenal/biliary colic

Obstetrics

Dose 10-20 mg 25-100 mg
ADRs ToleranceReleases histamines

Postpartum hemorrhage

Less toleranceLess release

None

Pharmacokinetics I/M I/M or I/V

Fentanyl:

Is highly lipid soluble and 80 times more potent than morphine, has short duration of action. It is used in anesthesia or as transdermal patch.

It also undergoes hepatic oxidation. 1st pass metabolism occurs in intestinal walls.

Drawback

Trunkal rigidity may occur –remifentanil, sufentanil even morphine

Increase in tone of large muscles, mediated at supraspinal level.

If drug is used in surgical anesthesia, thoracic compliance is reduced, interfering with ventilation.

Naloxone is not given, as have to maintain analgesia. Give cortical acting agent as we want to maintain analgesia.

Sufentanil and alfentanil are metabolized in liver.

Ramifentanil is metabolized by tissue esterases, also heroin (diacetyl morphine)

Partial agonists

Nalbuphine is given I/V and is very potent.

Pentazocine is commonly used along with others.

Buterphenol is weak partial agonist.

Pentazocine

Pentazocine is psychomimetic on kappa receptors, producing dysphoria. Although has very low dependence, but has potential. Effective potency is 20 times less. When combined with NSAIDS, hemodynamic actions occur leading to increased arterial blood pressure and heart load, with chances of tachycardia.

Not used for left ventricular failure. Other actions are less nausea and vomiting, less constipation and less miosis.

Adverse effects

If used continuously:

  1. Psychomimetic effects –hallucinations, unpleasant dreams
  2. No euphoria but dysphoria
  3. Can precipitate acute withdrawal symptoms in addicts

Nalbuphine

More potent than pentazocine with similar actions. Side effects are less. Hemodynamic effects are negligible. Analgesia occurs within 15 minutes.

Methadone

Weak long acting drug.

Is used in the treatment of patients addicted to opioids (heroin).

  • The idea behind methadone treatment is to replace the addict’s heroin with an orally active agonist that has a long duration of action.
  • This reduces drug craving & prevents symptoms of withdrawal.
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