MORPHINE SULFATE (AUTOINJECTOR)
Clinical safety rating: avoid
Positive evidence of fetus risks but benefits may outweigh risks in some cases
Morphine sulfate is a full opioid agonist that binds to mu-opioid receptors (MOR) in the central nervous system (CNS), inhibiting ascending pain pathways and altering pain perception and emotional response to pain. It also activates delta and kappa receptors, contributing to analgesia and side effects.
| Metabolism | Primarily hepatic via phase II glucuronidation (UGT2B7) to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). Minor pathways include N-demethylation to normorphine via CYP3A4 and sulfation. |
| Excretion | Primarily renal: 90% of morphine and its metabolites (morphine-3-glucuronide, morphine-6-glucuronide) are excreted in urine. About 7-10% is eliminated via bile and feces. Enterohepatic recirculation occurs. |
| Half-life | Terminal elimination half-life: 2-4 hours in adults; prolonged to 3-6 hours in elderly, neonates, or patients with hepatic/renal impairment. Clinical context: Half-life does not correlate directly with duration of analgesia; analgesic duration is 4-6 hours due to slow CNS equilibration. |
| Protein binding | 30-35% bound, primarily to albumin. Low protein binding minimizes drug interactions. |
| Volume of Distribution | 3-5 L/kg (approximately 210-350 L for a 70 kg person). High Vd indicates extensive tissue distribution, including skeletal muscle, kidneys, liver, and CNS. |
| Bioavailability | Intravenous: 100%. Intramuscular: 75-100% (mean 80%). Subcutaneous: 60-80%. Oral: 20-40% (extensive first-pass metabolism). Autoinjector (intramuscular): 80%. |
| Onset of Action | Intravenous: 2-5 minutes (peak effect 5-10 min). Subcutaneous: 10-15 minutes. Intramuscular: 10-30 minutes. Oral: 30-60 minutes. Autoinjector (intramuscular): 10-15 minutes. |
| Duration of Action | Intravenous: 2-4 hours. Subcutaneous/Intramuscular: 4-6 hours. Oral: 4-6 hours. Autoinjector (intramuscular): 4-6 hours. Clinical note: Duration may be shorter with repeated doses due to tolerance. |
Adult: 2–5 mg IV every 5–10 minutes as needed for pain; 10 mg IM every 4 hours as needed for pain. For autoinjector, typical dose is 5–10 mg IM or SC once.
| Dosage form | SOLUTION |
| Renal impairment | GFR 30–50 mL/min: reduce dose by 25–50% and extend dosing interval; GFR <30 mL/min: administer 50–75% of normal dose every 8–12 hours or consider alternative. Morphine is not recommended in GFR <15 mL/min without dialysis. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: reduce dose by 50% or extend interval. Child-Pugh C: avoid or reduce dose by 75% with extended intervals. |
| Pediatric use | Infants and children: 0.05–0.2 mg/kg IV/IM/SC every 2–4 hours as needed; maximum single dose 15 mg. For neonatal patients, use 0.025–0.1 mg/kg every 4–6 hours. |
| Geriatric use | Elderly patients: start at 2.5–5 mg every 4 hours IM/SC; reduce dose by 25–50% of adult dose due to increased sensitivity and risk of respiratory depression. Use with caution and titrate slowly. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
CNS depressants including alcohol and benzodiazepines increase sedation risk Life-threatening respiratory depression may occur.
| Breastfeeding | Morphine enters breast milk with M/P ratio of approximately 1.1; low relative infant dose (≤10% maternal weight-adjusted dose) is generally considered compatible with breastfeeding, but monitor infant for sedation and respiratory depression. |
| Teratogenic Risk | First trimester: Limited data, but no major malformations reported in human studies; animal studies show increased risk of neural tube defects at high doses. Second and third trimesters: Chronic use may cause fetal dependence and neonatal abstinence syndrome post-delivery; also associated with reduced fetal growth and preterm birth. |
■ FDA Black Box Warning
WARNING: ADDICTION, ABUSE, AND MISUSE; LIFE-THREATENING RESPIRATORY DEPRESSION; ACCIDENTAL INGESTION; NEONATAL OPIOID WITHDRAWAL SYNDROME; RISKS FROM CONCOMITANT USE WITH BENZODIAZEPINES OR OTHER CNS DEPRESSANTS; and RISK OF MEDICATION ERRORS. See full prescribing information for complete boxed warning.
| Common Effects | Constipation |
| Serious Effects |
["Hypersensitivity to morphine or any component","Significant respiratory depression","Acute or severe bronchial asthma in an unmonitored setting","Known or suspected gastrointestinal obstruction (including paralytic ileus)","Concurrent use of monoamine oxidase inhibitors (MAOIs) or within 14 days","Severe hypotension or hemodynamic instability","Increased intracranial pressure (relative, with careful monitoring)"]
| Precautions | ["Risk of respiratory depression, especially at initiation or dose escalation","Risk of opioid-induced hyperalgesia","Serotonin syndrome with concomitant serotonergic drugs","Adrenal insufficiency","Hypotension and orthostatic hypotension","Seizures in at-risk patients","Biliary tract spasm","Urinary retention","Severe injection site reactions (autoinjector)","Tolerance and physical dependence","Withdrawal on abrupt discontinuation"] |
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| Fetal Monitoring | Monitor maternal vital signs, pain scores, respiratory rate, and level of consciousness. Assess fetal heart rate and uterine activity if used during labor. Observe neonates for symptoms of opioid withdrawal (e.g., irritability, hypertonia, poor feeding) for 48-72 hours after delivery. |
| Fertility Effects | May impair fertility in both sexes. In females, chronic use can alter menstrual cycle and reduce fertility due to disruption of hypothalamic-pituitary-gonadal axis. In males, may lower testosterone levels, affecting sperm production and libido. |