What is Subcutaneous Morphine and How is it Administered?
Subcutaneous morphine is an injection given under the skin (not a patch), and it represents the preferred alternative route when patients cannot take oral medications. 1
Form and Route
Subcutaneous morphine is administered as a liquid injection beneath the skin, not as a transdermal patch. 1 The subcutaneous route is simpler and less painful than intramuscular injection, making it the standard parenteral approach for chronic cancer pain. 1
Administration Methods
Subcutaneous morphine can be given in two ways:
- Bolus injections every 4 hours using a butterfly needle or similar device 1, 2
- Continuous subcutaneous infusion using a syringe driver pump for patients requiring ongoing parenteral morphine 1, 2, 3
Dosing Conversion
The oral to subcutaneous potency ratio is approximately 1:2 to 1:3, meaning if a patient takes 20-30 mg orally, they would need only 10 mg subcutaneously to achieve equivalent pain relief. 1, 4 Most guidelines use the 1:2 ratio for practical conversion. 1, 4
Breakthrough Pain Management
For breakthrough pain, give the same dose as the regular 4-hourly subcutaneous dose, available as frequently as every hour if needed. 1, 2 The total daily morphine dose should be reviewed daily and adjusted based on how many rescue doses were required. 1, 2
Special Considerations for Elderly Hospice Patients with Renal/Hepatic Impairment
In patients with renal impairment, morphine's active metabolites (M3G and M6G) accumulate significantly and cause toxicity, including nausea, vomiting, and delirium. 5 All patients with elevated serum creatinine in one hospice study experienced opioid-related adverse effects, with significantly higher metabolite concentrations. 5
For patients with chronic kidney disease stages 4-5 (eGFR <30 ml/min), fentanyl or buprenorphine via transdermal or intravenous routes are the safest opioid choices rather than morphine. 1 Buprenorphine is particularly advantageous as it undergoes hepatic excretion and does not accumulate in renal failure, maintaining unchanged pharmacokinetics even in hemodialysis patients. 6
In hepatic impairment, morphine clearance decreases with corresponding increases in half-life, and the metabolite-to-morphine ratios decrease, indicating diminished metabolic activity. 7 If morphine must be used in renal or hepatic impairment, reduce doses and increase dosing intervals. 1
Practical Limitations
Subcutaneous administration may not be practical in patients with:
- Generalized edema 1, 2
- Coagulation disorders 4, 2
- Very poor peripheral circulation 1, 2
- Local site reactions (soreness, sterile abscesses, erythema) 4, 2
In such cases, intravenous administration becomes necessary, particularly in patients with existing IV access. 1
Clinical Effectiveness
Subcutaneous morphine via syringe driver has been shown to decrease dose frequency problems and improve control of pain and other symptoms in the last week before death, without evidence of shortening survival. 3 The subcutaneous route is safe and effective in intensive care and hospice settings, with minimal local irritation (only 3 episodes in 60 patient-days in one study). 8