From the August 21, 2003 issue

Problem: A community pharmacist accidentally dispensed the antithyroid medication propylthiouracil 50 mg instead of PURINETHOL (mercapto-purine) 50 mg, an antimetabolite for a child with acute lymphoblastic leukemia. His parents noticed that the tablets looked different, but the pharmacist mistakenly believed that a generic product existed and reassured the parents that it was the correct drug. The child received the wrong drug for 6 months. No harm occurred, but he missed 6 months of chemotherapy. Modifications in the therapy and numerous thyroid blood levels were needed.

This is one of several reports in which propylthiouracil was dispensed instead of mercaptopurine. Conversely, mercaptopurine has been dispensed and administered when propylthiouracil had been prescribed. Since propylthiouracil doses are often several hundred milligrams a day, mistakes that result in giving mercaptopurine at these high doses could lead to significant harm, including bone marrow suppression, hepat-otoxicity, immunosuppression, and teratogenicity. In one reported case, the patient developed pancytopenia and hepatotoxicity.

The two products are often located next to each other, contributing to the risk of an error. Name similarity also is a problem. Although the drug names appear to be quite distinct, there are several common characters that may lead to confusion: both names start with “P” and end with “L”; 50 mg tablet strengths are common to both; and phonetically, the “your” sound in “purine” and “uracil” increase the risk of an error. Also, propyl- thiouracil is often abbreviated “PTU,” which can be confused with “Purinethol” or “6MP,” a dangerous abbreviation sometimes used for mercaptopurine.

Safe Practice Recommendation: On several occasions, GlaxoSmithKline, the manufacturer of Purinethol, has sent alerts to pharmacists about the potential for this type of error. Along with their most recent alert in June 2003, they distributed “shelf shouters” that pharmacists can place wherever Purinethol is stored to remind staff about confusion with propylthiouracil and to confirm the indication with the patient. You might also consider affixing auxiliary labels to the drug containers or adding alerts to computers, especially if these drugs are not used frequently.

Never store these drugs in close proximity. Even if prescriptions have been properly entered into the computer, the incorrect product could be selected if the two medications are near each other. Pharmacies that use bar coding or match the drug container’s national drug code number against the one listed in the computer database (and printed along with the label) are less likely to select the wrong container.

Fully investigate patient-reported differences in tablet appearance. Some pharmacy computer systems can provide a picture of each tablet on the screen to help ensure accurate dispensing. Of course, patients should be counseled before either of these medications are dispensed in an outpatient setting; the counseling session could quickly alert a pharmacist to a potential mix-up. Prescribers can help avoid errors, too, by listing brand and generic names on prescriptions for Purinethol. Drug names should not be abbreviated; in particular, PTU and 6MP should never be used. If abbreviations are used, a pharmacist should always verify the order with the prescriber before dispensing the product.