A woman shows her clean syringes at the Aids Center of Queens County needle exchange outreach center in New York, November 28, 2006. Credit: Reuters/Shannon Stapleton

Reporting in the journal Addiction, researchers say that based on their study -- an analysis of five previous reviews of needle-exchange programs -- the evidence for the programs' effectiveness is weaker than generally thought.

However, they also stress that their review did not find needle-exchange programs to be ineffective either.

The findings of this review should not be used as a justification to close NSPs (needle and syringe programs) or hinder their introduction, write the researchers, led by Norah Palmateer, of Health Protection Scotland, part of the UK National Health Service.

Insufficient or weak evidence of an effect is not evidence of no effect, Palmateer told Reuters Health in an email. It is more a reflection of the studies and evidence available.

It is not that studies on needle-exchange programs have been poor, Palmateer said, but they are limited by the nature of their design.

Studies looking at needle-exchange programs have been observational, rather than controlled clinical trials where researchers would, for example, randomly assign some communities to start a program, and then compare them over time with program-free communities.

Observational studies, Palmateer noted, are subject to limitations like selection bias. For example, if those injection drug users at greatest risk of HIV are most likely to use the programs, then a study may find that program attendees have a higher rate of infection than drug users not involved in needle-exchange.

Needle-exchange programs have always been controversial, with opponents arguing that they sustain people's addictions and send the wrong message about drug use. The U.S. just recently repealed a ban on federal funding for needle-exchange programs, though some cities have long had their own programs.

Advocates of the programs, including many public-health and HIV experts, point to studies showing that needle and syringe exchange can cut HIV transmission -- such as a 2004 review by the World Health Organization (WHO) that concluded there is compelling evidence that the programs reduce HIV infections.

However, individual studies have come to mixed conclusions, including those covered by the WHO review, according to the current study.

Palmateer and her colleagues found that of the 10 studies in the WHO review focusing on HIV transmission, five had positive findings; of those five, four had weaknesses in their design that limit the conclusions that can be drawn.

Palmateer's team also looked at two other reviews that covered many of the same studies as the WHO review. One research team came to similar conclusions as the WHO, while the other was more guarded -- saying that the evidence that needle-exchange programs reduce HIV transmission is modest.

Overall, Palmateer and her colleagues conclude, there appears to be tentative evidence that needle-exchange programs reduce HIV transmission among injection-drug users.

When it came to hepatitis C, a liver infection usually spread through infected blood, there was insufficient evidence to say whether the programs are effective or not, according to Palmateer's team. Of the five reviews she and her colleagues analyzed, the three major ones did not examine hepatitis C in any depth, the researchers write.

There was also insufficient evidence of the effectiveness of alternatives to standard needle-exchange programs -- including vending machines that sell syringes and needles, and outreach programs that go to drug users rather than having them come to a clinic.

On the other hand, there was strong evidence across the reviews that needle-exchange programs reduce the sharing or reuse of dirty needles, and no evidence of harmful effects, according to Palmateer's team.

Exactly why the evidence for disease prevention is not as strong is not entirely clear. Studies may have failed to detect an impact, but limitations of the programs themselves may also be at work.

For example, many of the needle-exchange programs studied in these reviews had strict limits on the number of syringes and needles they could give clients, Palmateer and her colleagues note. So while they might have reduced users' needle sharing and reuse, it might not have been adequate.

It is not known what level of coverage -- that is, the amount of injecting equipment given to clients -- is needed to lower HIV and hepatitis C rates, according to Palmateer's team. And at any rate, the optimal level will vary from one locale to another.

The main public health implications of the findings are that a higher level of coverage of interventions, including (needle and syringe programs), is likely required to reduce blood-borne virus transmission, Palmateer said.

She noted that this may be especially true of hepatitis C, which is most commonly transmitted through drug-equipment sharing. In the U.S., injection drug use is believed to account for most new cases of hepatitis C and about one-fifth of new HIV cases.