Desperate heroin users in a few African cities have begun engaging in a practice that is so dangerous it is almost unthinkable: They deliberately inject themselves with another addict's blood, researchers say, in an effort to share the high or stave off the pangs of withdrawal. The practice, called flashblood or sometimes flushblood, is not common, but has been reported in Dar es Salaam, Tanzania, on the island of Zanzibar and in Mombasa, Kenya. It puts users at the highest possible risk of contracting AIDS and hepatitis. While most AIDS transmission in Africa is by heterosexual sex, the use of heroin is growing in some cities, and experts are warning that flashblood -- along with syringe-sharing and other dangerous habits -- could fuel a new wave of AIDS infections. 'Injecting yourself with fresh blood is a crazy practice -- it's the most effective way of infecting yourself with HIV,' said Dr. Nora D. Volkow, director of the National Institute on Drug Abuse, which supports the researchers who discovered the practice. 'Even though the number who do it is a relatively small group, they are vectors for HIV because they support themselves by sex work.' Sheryl A. McCurdy, a professor of public health at the University of Texas in Houston, first described the practice five years ago in a brief letter to The British Medical Journal and recently published a study of it in the journal Addiction. 'I don't really know how widespread it is,' said McCurdy, who is contacting other researchers working with addicts to get them to survey their subjects about it. 'There's pretty circular movement in East Africa, so I wouldn't be surprised if it's in other cities.' Whether someone can actually get a drug rush from such a relatively tiny amount of blood has never been tested, McCurdy said. Humans have about five quarts of blood, and the flashblood-user injects less than a teaspoon. 'They say they do,' she said. 'They pass out as if they just got a high. But I've talked to doctors who say that could be entirely the placebo effect.' One possibility, she said, is that traces of the drug are still in the syringe. After piercing a vein, an addict will typically draw some blood into the syringe, push it back out and repeat that three or four times to make sure all the heroin has been flushed into their blood. Those offering flashblood will usually hand over the syringe after only one in-out cycle. 