Which opioids can be administered intramuscularly (IM)?

Opioids Available for Intramuscular Administration

While intramuscular (IM) opioid administration is technically feasible for several agents, it is generally NOT recommended for chronic pain management because subcutaneous administration is simpler, less painful, and equally effective. 1

Opioids That CAN Be Given IM

Fentanyl

• FDA-approved for IM administration with onset of action occurring 7-8 minutes after injection and duration of 1-2 hours 2
• Available in concentrations of 50 mcg/mL in various ampule sizes 2
• Approximately 100 mcg IM fentanyl equals 10 mg morphine or 75 mg meperidine in analgesic activity 2

Morphine

• Can be administered IM, though this route is explicitly discouraged for chronic cancer pain 1
• When parenteral morphine is needed, subcutaneous route is preferred as it is simpler, less painful, uses smaller needles, has less risk of nerve damage, and allows easier visualization of veins to avoid inadvertent IV injection 1

Other Opioids for Parenteral Use

• Hydromorphone and diamorphine (in the UK) are preferred over morphine for parenteral administration due to greater solubility, allowing smaller injection volumes 1
• These agents can be given IM when parenteral administration is required 3

Clinical Context and Recommendations

When Parenteral Opioids Are Indicated

Patients presenting with severe pain requiring urgent relief should receive parenteral opioids via IV or subcutaneous routes—NOT intramuscular 1

The ESMO guidelines specifically state that for acute severe pain management, the preferred parenteral routes are:

• Intravenous (fastest onset)
• Subcutaneous (simpler, less painful than IM)
• Intramuscular is mentioned but not recommended as a primary route 1

Why IM Is Discouraged

Key reasons to avoid IM administration:

• More painful than subcutaneous injection 1
• Requires larger needles 1
• Higher risk of nerve damage 1
• More difficult to avoid inadvertent IV injection 1
• No therapeutic advantage over subcutaneous route 1

Preferred Alternative Routes

When oral administration is not possible, the hierarchy of preferred routes is:

1. Subcutaneous (first choice for most situations) 1
2. Intravenous (for patients with IV access, severe edema, coagulation disorders, poor peripheral circulation, or erythema/abscesses from subcutaneous injections) 1
3. Rectal (bioavailability equivalent to oral for morphine, 1:1 potency ratio) 1
4. Transdermal (fentanyl or buprenorphine for stable opioid requirements) 1

Practical Considerations

Dosing Conversions for Parenteral Routes

• Oral to subcutaneous morphine: approximately 3:1 ratio (divide oral dose by 3) 1
• Oral to IV morphine: approximately 3:1 ratio 1
• Individual titration may be needed after initial conversion 1

Common Pitfall

Do not default to IM administration simply because it is familiar or traditional—the evidence clearly supports subcutaneous as the superior parenteral route for most clinical scenarios requiring non-oral opioid delivery 1