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METHADONE, used to treat withdrawal symptoms in recovering heroin addicts, is known to be less effective if the patient is infected with HIV. Now Ulrich Hengge from the University of Essen in Germany and his colleagues say they know why. They found that HIV reduces the number of opioid receptors (OR) on white blood cells, which bind to methadone. The more viruses there were in the blood, the fewer receptors there were on the blood cells. Hengge suggests that monitoring OR levels could be a good way to track the progress of the HIV infection. Their findings will be published in the journal Pain.