Behind bars, a tattoo “needle” is a twist tie, paper clip or guitar string, slipped sometimes through the plastic casing of a Bic pen and heated up with the motor from a hair dryer or electric shaver. The same sharp point etches a gang symbol on an arm and a teardrop on a cheek; the ink is used again and again—and there’s a lot of blood.

Prison tattoos are as raw as they come (and known to spread hep B and maybe C), but the jury is still out on whether or not they’re a risk for HIV.

The murky debate is back in the news this week following a Canadian government show of support for a “safe” tattooing project shut down last month by another branch of the government. The sterile tattoo program “has demonstrated potential to reduce harm and enhance the health and safety of staff members, inmates and the general public,” according to the Correctional Service of Canada (CSC).

Minister of Public Safety Stockwell Day, the official behind the December shutdown, argued at the time that the $800,000 pilot project was a waste of taxpayers’ money and this week added a warning that inmates might use program needles to attack prison guards.

The program’s supporters report no rise in assaults and argue that it saves the government lots of money on treatment every time it prevents an HIV or hepatitis infection. An estimated 45% of Canadian prisoners find a way to get tattoos in prison even though they’re against the rules; the U.S. numbers are probably about the same. “Let’s just accept that these activities are going on and do our best to reduce the harm,” argues Leon Mar of the Canadian HIV/AIDS Legal Network.

Canada’s safe tattoo pilot program appears to be something of a political football between the Liberal Party administration that left office last year after requesting the program be evaluated and the governing Conservative Party that shut it down. But the fact is that neither side in this debate can count on any hard science.

The U.S. Centers for Disease Control (CDC) website states: “A risk of HIV transmission does exist if instruments contaminated with blood are either not sterilized or disinfected or are used inappropriately between clients.” And former prisoner Jason Farrell, who runs Harm Reduction Consulting Services, says, “The dangers don’t lie in the actual tattoo; it’s the hidden danger of blood splattering and not being able to create a sterile environment.” Yet the CDC’s Jennifer Ruth told, “The CDC can’t trace a single case of HIV transmission to a dirty tattoo needle.”

A Georgia prison study on HIV transmission last year shed little light on this issue. In the study, 12 HIV positive inmates listed unsafe tattooing as their only risk factor. But this kind of “self-reporting” has its flaws. First-person accounts make for shaky data anyway, and in this case, prisoners may have good reason to lie: The alternative explanations for how they got HIV are less socially acceptable.

“People are more likely to admit to nonstigmatized behaviors like tattooing rather than IV drug use,” says Ian Williams, PhD, of the CDC’s Epidemiology Division of Viral Hepatitis. Likewise, many inmates would rather not admit to having had sex with men.

Who’s to say where a prisoner picks up the virus—without knowing more about HIV and life behind bars? Supporters of the Canadian program say the lack of such information is yet another argument on their behalf. “The initiative has resulted in an enhanced level of knowledge and awareness amongst staff and inmates regarding blood-borne infectious disease prevention and control practices,” according to the CSC report.

Not that science always dictates prevention policy inside or outside prison walls. While condoms were long ago proved to help stop the spread of HIV—and are distributed in prisons in Canada, Australia and many European countries—they’re still not allowed in a vast majority of U.S. prisons.