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 H.I.V.,” 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 H.I.V. because they support themselves by sex work.”