Which of the following is the most appropriate conversion to methadone for a patient who received 100 mg of IV morphine yesterday over 24 hours?
a) Methadone 100 mg IV
b) Methadone 100 mg PO
c) Methadone 25 mg PO
d) Methadone 10 mg IV
e) Cannot answer without more information
Answer e. The conversion from morphine to methadone, once thought to be 1:1, is probably more like 1:0.25 or even 1:0.1. Interestingly, it may also depend on how high the dose of morphine is, for how long the exposure to morphine has taken place, and on the degree of cross-tolerance (which may vary). The safest way to complete this conversion is to start on the low end and titrate the dose up, observing the patient for signs of increased (and unanticipated) pharmacologic effect.