Dosage Calculations and Conversions
Dr. Abrams offered these suggestions for dealing with dosages:
▸ In switching a patient from one opioid to another, rely on published equivalence charts but lower the calculated dose by one-third to be safe. Different opioids bind slightly differently to receptors, and incomplete cross-tolerance is common, she noted.
▸ The conversion from morphine to methadone depends on whether the patient is on a low dose or a high dose. At low doses of morphine–30 mg per day or so–4 mg of morphine is equivalent to 1 mg of methadone. But at over 1,000 mg of morphine per day, 20 mg of morphine is equivalent to 1 mg of methadone.
▸ When switching a patient from intravenous to oral Dilaudid (hydromorphone hydrochloride), increase the dose by a factor of five. This seems high, but the fact is that the liver metabolizes four-fifths of the oral Dilaudid on its first pass. “Anybody who has been getting 2 mg of intravenous Dilaudid and then gets 4 mg by mouth, you have convinced him that only the shots work,” Dr. Abrahm said.
▸ In converting a patient from oral to intravenous morphine–such as when the patient is coming into the hospital with nothing by mouth–don't forget to include rescue doses in the calculation. If the patient is taking 150 mg of sustained-release morphine every 12 hours, and his or her rescue oral opioid totals 150 mg/day, the total 24-hour morphine dose is 450 mg. That's equivalent to 150 mg IV or 6.25 mg/hr. “It doesn't hurt to put this [calculation] on the chart,” she said, because 6.25 mg/hr seems like a lot to hospital staff.
If this isn't done, patients will normally be started at 1 mg/hr, and Dr. Abrahm said that she has experienced patients going into withdrawal before she could write the higher dose on the chart. The rescue dose should be 10% of the total daily dose every 4 hours. For this patient, that's 15 mg every 4 hours, or more commonly, 5–10 mg every 2 hours.