First-ever opioid conversion text released

August 31, 2009

Want to see fear on the face of a pharmacist? Ask for a quick dosage conversion for a 75-year-old woman with breast cancer who has become too weak to swallow MS Contin tablets and needs an oral liquid morphine. Keep reading to learn more about making conversions easier.

Want to see fear on the face of a pharmacist? Ask for a quick dosage conversion for a 75-year-old woman with breast cancer who has become too weak to swallow MS Contin tablets (controlled-release morphine, Purdue Pharma) and needs an oral liquid morphine. By the way, she also needs escalating doses of morphine to maintain analgesia, she has breakthrough pain, and her kidneys are failing.

And what will be the next switch when she becomes unable to swallow the liquid formulation? “People typically handle opioid conversions badly,” said Mary Lynne McPherson, PharmD, BCPS, CDE, hospice and palliative care consultant and professor of pharmacy practice and science at the University of Maryland Baltimore School of Pharmacy.

“Patients get into trouble every day because they are either overdosed or underdosed when an opioid conversion goes wrong. There was nothing out there to guide them except a few scattered articles, so I sat down and wrote the book.”

The book she wrote, “Demystifying Opioid Conversion Calculations,” is the first text devoted exclusively to opioid conversions, said Laura Scarpaci, manager of clinical performance at excelleRx, a Philadelphia-based hospice and long-term care pharmacy. The new volume is published by the American Society of HealthSystem Pharmacists and is available at ASHP’s online bookstore.

“We pharmacists are faced with nurses and physicians asking, sometimes pleading, for help in converting opioid doses,” Scarpaci said. “There are paper conversion tables and online calculators, but they are only a starting point. There are a lot of patient variables that have to be taken into account when converting from one drug to another and from one route of administration to another.”

And you just can’t plug those variables into a calculator.

“Looking back at pharmacy school, it would have been helpful to have had a text like this,” she said. “If you weren’t lucky enough to have Lynn McPherson as a professor like I was, I don’t know how you could pick up this much practical information in a concise source.”

While McPherson’s focus is on hospice and palliative care, opioid conversions can be a problem in any pharmacy setting.

Community pharmacists may need to convert patients from a combination agent such as Percocet (oxycodone/acetaminophen, Endo) to a long-acting morphine.

Hospital pharmacists may need to convert patients from an IV opioid to an oral formulation on discharge.

Long-term care pharmacists may need to deal with escalating opioid tolerance and declining physical status that can affect both administration and drug metabolism.

“There is a real art to doing conversions,” said Eliot Cole, MD, executive director of the American Society of Pain Education. “When conversions go bad, it is usually because of a lack of appreciation for the nuances that affect every patient on an individual basis.”

The key weakness of existing opioid conversion guides is the methodology used to study conversions, Dr. Cole said. Most conversion tables and calculators are based on single-dose crossover studies of analgesia in healthy volunteers. But in the real world of patient care, patients who need conversion calculations are not taking single doses and they are not healthy.

Drug math calculators cannot factor in key variables such as patient age, health status, renal function, opioid tolerance, metabolites, subjective pain relief, adverse effects, and other individual factors. There are also factors unique to specific drugs that can affect conversion decisions. Methadone is an extremely effective and affordable analgesic, Scarpaci said. But many prescribers shy away from methadone because they consider it dangerous.

The reality is that methadone is no more dangerous than other opioids; it is just different, she said. And the typical table that shows a 2:1 conversion from morphine to methadone is seldom accurate in practice. Methadone has a half-life of 130 to 150 hours, which makes single-dose crossover studies dubious at best, McPherson said. Patients must be monitored for four to six days after conversion from other agents.

“Too many people fail to recognize the effect that long half-life can have on patients,” McPherson said. “It’s a wonderful analgesic with multiple mechanisms of action and it is highly cost-effective. But people are fearful of using it because they don’t understand it.”

Understanding the factors that can affect opioid conversions is the key to getting conversions right, Dr. Cole said. If the new dose looks wrong, given the patient’s condition, it’s time to try a new approach. “Lynn writes with humor,” he said, “but what she covers is lethally serious. If you don’t get it right, you will kill somebody.”