Dosing

Dosing

39. What dose of methadone should be prescribed to treat A.J.'s pain?

To convert A.J.'s dose of morphine to an equivalent dose of methadone, use Table 8-3 as a standard reference for comparison. A.J.'s 10-mg every hour oral morphine sulfate solution dose is estimated to be approximately equivalent to 10 mg every 3 hours of morphine IM. On a single-dose basis, IM doses of methadone and morphine are equally potent; however, with repeated doses on a chronic basis, the duration of methadone activity is approximately four times longer than that of morphine. Therefore, the total daily parenteral methadone dose will be only one-fourth that of parenteral morphine. In this case, A.J. would require 20 mg/day of IM methadone. Because oral methadone is only 75% as effective as methadone IM,¹⁴³ A.J. should receive approximately 7.5 mg of methadone orally every 6 hours. Approximately 5 to 10 days will elapse before the full methadone effect is realized, unless loading doses equal to twice that of the maintenance analgesic dose are used for the first two doses. These two larger doses accelerate the process of reaching the steady-state concentration. Breakthrough pain should be anticipated despite around-the-clock dosing of methadone, especially immediately after the conversion from other opioids. Because of long half-life and risk of accumulation, additional methadone doses should not be used as a breakthrough drug. Instead, morphine or oxycodone would be reasonable choices to treat A.J.'s breakthrough pain before methadone reaches its steady-state and maintenance dose can be safely increased.

Posted in: General Care,Pain and Its Management