Alternative Injectable Opioids When Injectable Morphine is Unavailable
For a patient on oral morphine 10mg every 4 hours who requires parenteral administration, use injectable hydromorphone or diamorphine as the preferred alternatives, with a conversion ratio of oral morphine to subcutaneous morphine of approximately 1:3 (meaning 3-5mg subcutaneous morphine equivalent every 4 hours). 1
Primary Injectable Alternatives
Hydromorphone (First-Line Alternative)
- Hydromorphone is specifically recommended as a preferred alternative for parenteral administration because it is 5-10 times more potent than morphine and significantly more soluble, allowing smaller injection volumes. 1
- For your patient on oral morphine 10mg q4h (60mg/day total), convert to approximately 20mg/day parenteral morphine equivalent, then calculate hydromorphone dose at 1/7th to 1/10th of this amount. 1
- Hydromorphone has similar efficacy and adverse effect profile to morphine when used in equianalgesic doses. 1
Diamorphine (UK and Select Countries)
- Diamorphine is the preferred injectable opioid in Britain due to considerably greater solubility than morphine, ensuring injection volumes remain small. 1
- Diamorphine is approximately 1.5 times more potent than morphine by mouth. 2
- The oral morphine to parenteral diamorphine conversion follows similar 1:3 ratio principles as morphine. 2
Dosing Conversion Strategy
Calculate Parenteral Dose from Oral Morphine
- Divide the total daily oral morphine dose by 3 to obtain the parenteral equivalent dose, then adjust upward or downward based on individual response. 1
- For oral morphine 10mg q4h = 60mg/day oral → approximately 20mg/day parenteral morphine equivalent. 1
- This converts to roughly 3-4mg subcutaneous every 4 hours as starting dose. 1
Route Selection
- Subcutaneous administration is strongly preferred over intramuscular because it is simpler, less painful, requires smaller needles, and has similar absorption with peak plasma concentrations within 15-30 minutes. 1, 3
- Intravenous administration should be reserved for patients with: existing IV access, generalized edema, coagulation disorders, poor peripheral circulation, or problems with subcutaneous sites (erythema, soreness, sterile abscesses). 1, 3
Non-Injectable Alternatives to Consider First
Rectal Administration
- Rectal morphine has 1:1 bioavailability with oral morphine, meaning the same 10mg dose every 4 hours can be used rectally with equivalent duration of effect. 1, 3
- Only use immediate-release formulations rectally—never controlled-release tablets. 3
- This route may be preferred by some patients and avoids injection-related complications. 1
Transdermal Fentanyl
- Transdermal fentanyl is recommended as a useful non-invasive alternative for patients with stable opioid requirements, though it is less flexible than shorter-acting preparations. 1
- Onset takes 8-16 hours with steady state at 72 hours, making it unsuitable for acute conversion or unstable pain. 1
- Each patch lasts 3 days, and the intradermal depot means serum levels take 16 hours to drop by 50% after removal. 1
Critical Pitfalls to Avoid
Conversion Errors
- The oral to parenteral potency ratio varies between 1:2 and 1:3 depending on individual patient factors—always start conservatively and titrate based on response. 1
- Provide breakthrough doses equal to the regular 4-hourly dose, available as often as every hour for subcutaneous route. 3
Route-Specific Contraindications
- Do not use intramuscular administration for chronic pain—it offers no advantage and causes more pain than subcutaneous. 1, 3
- Avoid buccal, sublingual, and nebulized routes as morphine absorption is unpredictable with no clinical advantage. 1, 3, 4