Syringe exchange programs to combat the spread of AIDS among intravenous drug users are effective but no panacea, participants of the first national conference on the subject were told Thursday.

The three-day North American Syringe Exchange Convention was held in Tacoma, where the nation's first non-underground needle exchange program was begun in 1988 by David Purchase, a private citizen, and later adopted by the local county health department.Needle exchanges inspired by Purchase's program are currently operating in Seattle, Spokane and Vancouver, Wash.; Portland, Ore.; New Haven, Conn.; Boulder, Colo.; and Honolulu in the United States, as well as in Victoria, Toronto and Vancouver, Canada, and many countries in Europe.

They have also been attempted in New York, Los Angeles and Boston but have been banned by anti-drug paraphernalia laws.

Don Des Jarlais, director of research of the Beth Israel Chemical Dependency Institute of New York, said in the conference's keynote address that statistical studies show it takes most drug addicts a year or more before they consider themselves full-fledged users and begin thinking about the health risks of sharing needles.

Beyond that, though, needle exchanges - in which drug addicts can obtain sterile hypodermics by turning in their used or "dirty" syringes - are remarkably effective, Des Jarlais.

A strict statistician Des Jarlais noted it is difficult given the lack of federal support for research and problems with confidentiality to prove that needle exchanges actually prevent the spread of AIDS. But, he said, they have proven effective in reducing "risk behavior," i.e. the sharing of needles, which is known to be a leading cause of the spread of the HIV virus which can cause AIDS.