Can I overdose by adding hydromorphone 0.5 mg 1 hour after morphine 4 mg?

Risk of Overdose When Adding Hydromorphone 0.5 mg One Hour After Morphine 4 mg

Yes, adding hydromorphone 0.5 mg one hour after morphine 4 mg creates a significant risk of overdose due to the additive respiratory depressant effects of these potent opioids.

Understanding the Risk Factors

When combining opioids, several important considerations determine the overdose risk:

1. Potency Relationship: Hydromorphone is significantly more potent than morphine:

   • Hydromorphone is approximately 5 times more potent than morphine orally
   • Hydromorphone is approximately 8.5 times more potent than morphine intravenously 1

2. Timing of Administration:

   • One hour after morphine administration, the first dose is still actively exerting effects
   • Morphine reaches peak respiratory depression later than hydromorphone (5.5 ± 2.8 vs. 3.0 ± 1.5 hours after infusion start) 2

3. Additive CNS Depression:

   • The FDA label for hydromorphone explicitly warns that "taking hydromorphone with other opioid medicines can cause severe drowsiness, decreased awareness, breathing problems, coma, and death" 3

Clinical Implications

Equivalent Dosing

If we consider intravenous administration:

• Morphine 4 mg is approximately equivalent to hydromorphone 0.47-0.8 mg (using 5:1 to 8.5:1 conversion ratios)
• Adding hydromorphone 0.5 mg to morphine 4 mg effectively doubles the opioid load

Safety Guidelines

Clinical guidelines recommend:

• For patients already receiving an opioid infusion who develop pain, it is reasonable to give a bolus dose equal to or double the hourly infusion rate 4
• However, this applies to the same opioid, not adding a different, more potent opioid

Respiratory Depression Risk

• Both opioids cause respiratory depression with different time courses
• Hydromorphone causes a more rapid onset of respiratory depression (respiratory nadir at 3.0 ± 1.5 hours) compared to morphine (5.5 ± 2.8 hours) 2
• The combined effect could lead to dangerous respiratory depression as both drugs reach peak effect

Safer Alternatives

1. Use a single opioid: Continue with morphine alone and titrate as needed

   • If a patient receives two bolus doses of the same opioid in an hour, guidelines suggest it is reasonable to double the infusion rate 4

2. Allow adequate time between doses:

   • Wait until the first opioid's effect has been properly assessed before adding another

3. Proper conversion:

   • If switching from morphine to hydromorphone, use proper conversion ratios (approximately 5:1) and reduce the calculated dose by 25-50% due to incomplete cross-tolerance 5

Conclusion

Adding hydromorphone 0.5 mg just one hour after morphine 4 mg administration creates a significant risk of overdose, particularly respiratory depression. This practice should be avoided in favor of using a single opioid with appropriate titration based on patient response.