What is the recommended starting dose of morphine (opioid analgesic) subcutaneously (SQ) for a 74-year-old female patient with cancer and impaired renal function, weighing 46 kg?

Subcutaneous Morphine Dosing for a 74-Year-Old Female Cancer Patient

The recommended starting dose of subcutaneous morphine for a 74-year-old female cancer patient weighing 46 kg is 5 mg every 4 hours, with the same dose available for breakthrough pain. 1

Dosing Considerations

• Subcutaneous administration is the preferred alternative route when patients are unable to take morphine orally 1
• The average relative potency ratio of oral morphine to subcutaneous morphine is between 1:2 and 1:3 (i.e., 20-30 mg oral morphine is equianalgesic to 10 mg subcutaneous morphine) 1
• For elderly patients with low body weight (46 kg), a conservative starting approach is warranted to minimize adverse effects while still providing pain relief 2
• The dose should be titrated against effect, with daily review of the total amount of breakthrough medication used 1

Administration Method

• Subcutaneous morphine can be given as intermittent injections (every 4 hours) or as a continuous infusion 1
• For patients requiring continuous parenteral morphine, subcutaneous infusion is the preferred method of administration 1
• Breakthrough doses should be available and can be offered as frequently as every 15-30 minutes for parenteral routes 1

Dose Titration

• The total daily dose of morphine should be reviewed daily and adjusted based on the amount of breakthrough medication required 1
• If more than four breakthrough doses are necessary in 24 hours, the baseline opioid treatment should be adjusted 1
• Steady state is achieved within 24 hours after starting treatment or following dose adjustment 1

Special Considerations for This Patient

• Age (74 years) and low body weight (46 kg) are factors that may increase sensitivity to opioids, suggesting a conservative initial dosing approach 2
• Renal function should be monitored closely in elderly cancer patients, as morphine metabolites can accumulate in renal impairment 3
• If the patient develops intolerable side effects on morphine, consider switching to fentanyl or sufentanil, which may be better tolerated in patients with renal impairment 4, 3

Managing Side Effects

• Common side effects include constipation (requires prophylactic laxative use), nausea, vomiting, and central nervous system effects (drowsiness, confusion) 1
• Initial drowsiness, dizziness, or mental clouding commonly occur at the start of treatment but typically resolve within a few days when patients are stabilized on a dose 1
• If side effects become intolerable, consider:
  • Reducing the opioid dose if possible 1
  • Using adjuvant medications to manage specific side effects 1
  • Switching to another opioid 1, 4

Pitfalls and Caveats

• Avoid intramuscular administration for chronic cancer pain as subcutaneous administration is simpler and less painful 1
• Be cautious with morphine in patients with renal impairment as active metabolites may contribute to toxicity 1
• Monitor elderly patients closely for signs of opioid toxicity, particularly with concomitant conditions like COPD or renal impairment 5
• Remember that the goal is to achieve "no worse than mild pain" (≤30/100 mm on a visual analog scale), which is achievable in the majority of patients with appropriate titration 6