Opioid Dose Conversions-focus on methadone
Resources:
Global RPh- has an automated narcotic conversion calculator. The only tricky part is the "Reduction for incomplete cross tolerance" field. This is a calculation that takes into consideration the concept of starting with a less than equianalgesic dose to avoid overdosing the patient. Many clinicians start with a 25-50% dose reduction.
American Family Physician article on using methadone for chronic pain treatment. A concise article with easy-to-use dosing tables.
OHSU opioid management document-has some dose conversion charts, but is harder to use. This does not include information on methadone dosing.
DOSE CONVERSIONS:
CAUTION: methadone conversions are non-linear. Again, the American Family Physician article gives a good example of this. (One way to think of this non-linear pattern is to think that morphine and other opioids "top out" more easily than methadone, while methadone keeps its potency at very high doses.)
Morphine:Methadone conversion ratio for-
under 100 mg morphine 3:1 , or, 33% of morphine dose
100-300mg morphine 5:1 , or, 20% of morphine dose
300-600 mg morphine 10:1 , or, 10% of morphine dose
600-800 mg morphine 12:1 , or, 8.3% of morphine dose
800-1000 mg morphine 15:1 , or, 6.7% of morphine dose
over 1000 mg morphine 20:1 , or, 5% of morphine dose.
Equianalgesic doses of other opioids:
30 mg morphine = 30 mg hydrocodone = 7.5 mg hydromorphone = 20 mg oxycodone
TO CALCULATE: add up the total daily dose of opioid, convert it to morphine equivalents, and multiply it by the % in the table above. Reduce this about 25% to get a starting dose. Divide this into 3 daily doses, to be taken every 8 hours.
EXAMPLE #1: Conversion of Vicodin to methadone: Your patient is taking 2 vicodin (5/500) four times daily for a total of 40 mg hydrocodone per 24 hours.
The equianalgesic dose of methadone is 13.2 mg (=40 x 0.33). You decide to start at about 75% of that dose, or 10 mg methadone/24 hours.
The usual starting dose schedule is q 8 hours, and methadone comes as 5 mg, 10 mg, and 40 mg tablets. 10 mg is not conveniently divided among three doses.
Your patient has the highest pain level in the late afternoon, so you decide to dose 2.5 mg q am (about 7 am for this patient), 5 mg at about 3 pm, and 2.5 mg q hs.
You counsel the patient to not expect the full level of pain relief from methadone during the first week, and they may need vicodin for breakthrough pain during this time. You schedule a follow-up in 1 week.
TO MONITOR: Pain levels (pain diary can help), sedation, constipation.
Methadone drug interactions:
Partial opiate agonists: these knock the methadone off of the receptors and can precipitate withdrawl symptoms.
CYP 3A4 inhibitors can increase the systemic methadone levels.
Documented drug interactions (not a comprehensive list):
Increase methadone effects: ciprofloxacin, diazepam, ethanol, fluconazole, urinary alkalinizers.
Decrease methadone effects: phenobarbital, phenytoin, rifampin, many anti-HIV drugs, and urinary acidifiers.
Global RPh- has an automated narcotic conversion calculator. The only tricky part is the "Reduction for incomplete cross tolerance" field. This is a calculation that takes into consideration the concept of starting with a less than equianalgesic dose to avoid overdosing the patient. Many clinicians start with a 25-50% dose reduction.
American Family Physician article on using methadone for chronic pain treatment. A concise article with easy-to-use dosing tables.
OHSU opioid management document-has some dose conversion charts, but is harder to use. This does not include information on methadone dosing.
DOSE CONVERSIONS:
CAUTION: methadone conversions are non-linear. Again, the American Family Physician article gives a good example of this. (One way to think of this non-linear pattern is to think that morphine and other opioids "top out" more easily than methadone, while methadone keeps its potency at very high doses.)
Morphine:Methadone conversion ratio for-
under 100 mg morphine 3:1 , or, 33% of morphine dose
100-300mg morphine 5:1 , or, 20% of morphine dose
300-600 mg morphine 10:1 , or, 10% of morphine dose
600-800 mg morphine 12:1 , or, 8.3% of morphine dose
800-1000 mg morphine 15:1 , or, 6.7% of morphine dose
over 1000 mg morphine 20:1 , or, 5% of morphine dose.
Equianalgesic doses of other opioids:
30 mg morphine = 30 mg hydrocodone = 7.5 mg hydromorphone = 20 mg oxycodone
TO CALCULATE: add up the total daily dose of opioid, convert it to morphine equivalents, and multiply it by the % in the table above. Reduce this about 25% to get a starting dose. Divide this into 3 daily doses, to be taken every 8 hours.
EXAMPLE #1: Conversion of Vicodin to methadone: Your patient is taking 2 vicodin (5/500) four times daily for a total of 40 mg hydrocodone per 24 hours.
The equianalgesic dose of methadone is 13.2 mg (=40 x 0.33). You decide to start at about 75% of that dose, or 10 mg methadone/24 hours.
The usual starting dose schedule is q 8 hours, and methadone comes as 5 mg, 10 mg, and 40 mg tablets. 10 mg is not conveniently divided among three doses.
Your patient has the highest pain level in the late afternoon, so you decide to dose 2.5 mg q am (about 7 am for this patient), 5 mg at about 3 pm, and 2.5 mg q hs.
You counsel the patient to not expect the full level of pain relief from methadone during the first week, and they may need vicodin for breakthrough pain during this time. You schedule a follow-up in 1 week.
TO MONITOR: Pain levels (pain diary can help), sedation, constipation.
Methadone drug interactions:
Partial opiate agonists: these knock the methadone off of the receptors and can precipitate withdrawl symptoms.
CYP 3A4 inhibitors can increase the systemic methadone levels.
Documented drug interactions (not a comprehensive list):
Increase methadone effects: ciprofloxacin, diazepam, ethanol, fluconazole, urinary alkalinizers.
Decrease methadone effects: phenobarbital, phenytoin, rifampin, many anti-HIV drugs, and urinary acidifiers.
posted by Martha at 12:57 PM
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