We have evaluated the ability of various opioid agonists, including methadone, L-alpha-acetylmethadol (LAAM), fentanyl, meperidine, codeine, morphine, and buprenorphine, to block the cardiac human ether-a-go-go-related gene (HERG) K(+) current (I(HERG)) in human cells stably transfected with the HERG potassium channel gene. Our results show that LAAM, methadone, fentanyl, and buprenorphine were effective inhibitors of I(HERG), with IC(50) values in the 1 to 10 microM range. The other drugs tested were far less potent with respect to I(HERG) inhibition. Compared with the reported maximal plasma concentration (C(max)) after administration of therapeutic doses of these drugs, the ratio of IC(50)/C(max) was highest for codeine and morphine (>455 and >400, respectively), thereby indicating that these drugs have the widest margin of safety (of the compounds tested) with respect to blockade of I(HERG). In contrast, the lowest ratios of IC(50)/C(max) were observed for LAAM and methadone (2.2 and 2.7, respectively). Further investigation showed that methadone block of I(HERG) was rapid, with steady-state inhibition achieved within 1 s when applied at its IC(50) concentration (10 microM) for I(HERG) block. Results from "envelope of tails" tests suggest that the majority of block occurred when the channels were in the open and/or inactivated states, although approximately 10% of the available HERG K(+) channels were apparently blocked in a closed state. Similar results were obtained for LAAM. These results demonstrate that LAAM and methadone can block I(HERG) in transfected cells at clinically relevant concentrations, thereby providing a plausible mechanism for the adverse cardiac effects observed in some patients receiving LAAM or methadone.