Pharmacology of Morphine


Morphine, named after the Greek god of dreams, Morpheus, is a prototype opioid agonist with potent analgesic properties. This article explores the pharmacology, pharmacokinetics, and side effects of morphine.

Table 1: Effects of Morphine in Humans

AnalgesiaRelief from pain
EuphoriaA sense of well-being and happiness
SedationInduces drowsiness
Diminished ConcentrationImpaired ability to focus
NauseaFeeling of queasiness
Body WarmthSensation of warmth in the body
Heaviness of ExtremitiesFeeling of weight in limbs
Dry MouthReduced salivation
PruritusItching, especially around the nose
Modified Pain PerceptionAlters the perception of pain
DysphoriaFeeling of unease or dissatisfaction (in the absence of pain)

Pharmacokinetics of Morphine

Table 2: Pharmacokinetic Parameters of Morphine

AbsorptionWell absorbed after intramuscular (IM) administration with onset in 15-30 minutes
Peak EffectPeak effect achieved in 45-90 minutes
Clinical Duration of ActionApproximately 4 hours
Oral AdministrationLimited by significant first-pass metabolism in the liver, bioavailability around 25%
Intravenous (IV) AdministrationPreferred for perioperative use
InhalationUsed to relieve dyspnea, but can depress ventilation
Blood-Brain Barrier TransitDelayed compared to some other opioids

Plasma Concentrations and Effects

Table 3: Plasma Concentrations and Effects of Morphine

Plasma Concentrations after IV InjectionPoor correlation with pharmacologic activity
Cerebrospinal Fluid (CSF) ConcentrationsPeak in 15-30 minutes after IV injection, decays more slowly than plasma
Minimum Plasma Concentration for AnalgesiaAt least 0.05 mg/mL for moderate analgesia

Metabolism of Morphine

Table 4: Metabolism of Morphine

Conjugation with Glucuronic AcidPrimary metabolism in hepatic and extrahepatic sites
MetabolitesMorphine-3-glucuronide (75-85% of dose) and Morphine-6-glucuronide (5-10%)
Renal MetabolismSignificant contribution to metabolism
EliminationPrimarily in urine (7-10% via biliary excretion)
Excretion of Unchanged DrugMinimal (1-2% in urine)

Table 5: Effects of Morphine Metabolites

MetabolitePharmacological Activity
Morphine-6-glucuronideAnalgesic and ventilatory depressant

Factors Affecting Morphine’s Effects

Table 6: Factors Affecting Morphine’s Effects

FactorInfluence on Morphine Effects
Blood pHAlkaline conditions enhance CNS penetration
Carbon DioxideHypercarbia increases brain concentration
Renal DysfunctionImpaired elimination may lead to accumulation

Gender Differences

Table 7: Gender Differences in Morphine Effects

Gender DifferenceEffect on Morphine
Analgesic PotencyGreater potency in women
Speed of OffsetSlower offset in women
Postoperative Opioid UseHigher in men
Ventilatory Response to CO2Decreased slope in women, no effect in men
Apneic ThresholdNo effect in women, increased in men
Hypoxic SensitivityDecreased in women, no effect in men

Side Effects of Morphine
Common side effects of morphine include:

  • Nausea and vomiting
  • Sedation and drowsiness
  • Respiratory depression
  • Constipation
  • Itching and pruritus
  • Tolerance and physical dependence with prolonged use

Morphine, a potent opioid agonist, offers effective pain relief but is associated with several side effects, including respiratory depression. Understanding its pharmacology and pharmacokinetics is essential for safe and effective use in clinical settings. Careful consideration of patient factors, such as gender and renal function, is crucial in tailoring morphine therapy.


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