The anticonvulsant Dilantin (phenytoin) can drastically reduce levels of the HIV drug efavirenz—found in Sustiva/Stocrin and Atripla—in the blood when both medications are used together, according to an article published by AIDSmap reviewing a case report in the January 2 issue of AIDS.

Cedric Spak, MD, from the University of Washington, Harborview Medical Center-Madison Clinic in Seattle, and his colleagues describe treating a man with a history of epilepsy who was diagnosed with HIV after presenting with oral thrush and wasting syndrome. The patient, who’d been taking Dilantin to manage his seizures, was first given Diflucan (fluconazole) to treat his thrush and other medications to prevent MAC and PCP. Because Diflucan can interact with Dilantin, his Dilantin dose was reduced. Once his thrush improved and his weight increased, Spak’s team decided to initiate antiretroviral therapy with a regimen containing Sustiva.

Though it was suspected that Dilantin could potentially reduce Sustiva blood levels, Spak’s team was more concerned about the Rifadin (rifampin)—an antibiotic that can reduce Sustiva levels by up to 30 percent—which he was receiving to prevent MAC. In turn, the team prescribed an 800 mg once-daily dose of Sustiva—200 mg higher than the standard dose.

Blood levels of Sustiva were monitored, and on the fourth and the 14th day of treatment were found to be undetectable. Despite this, however, the patient’s viral load had fallen from a high of over 1 million copies to just 12,400 copies, likely due to the Truvada (tenofovir and emtricitabine) he was taking in combination with Sustiva. After learning of the day 14 results, Spak’s team switched the patient from Dilantin to another anticonsulsive drug, and further increased the Sustiva dose to 600 mg twice a day. Two weeks later, the amount of Sustiva in the patient’s blood had approached normal levels and his viral load had fallen further to 921 copies. At this point, the team lowered the patient’s dose of Sustiva to the standard 600 mg once daily.

While the discontinuation of Dilantin resulted in a return of Sustiva blood levels toward normal, it is possible that the patient’s concurrent use of Rifadin made any potential interaction worse. Also, Spak’s team is quick to point out that because blood levels of Sustiva can vary a great deal among different people, the results seen in this patient cannot be generalized. News of this interaction, however, should advise doctors to proceed with caution when treating patients who are HIV positive and have seizure disorders.