There are tricky practical and legal barriers to fulfilling Conservative election promises aimed at making Canada's prisons drug-free, indicates an internal government study prepared for the public safety minister.

"No single international prison system has been able to eradicate drugs from prison," says a memo to Vic Toews about the findings from deputy minister Bill Baker, who has since retired.

"(The Correctional Service) has made a great deal of progress in terms of both interdiction and programming to address the issue of drug and alcohol use and addiction in prisons, but demand for drugs remains strong."

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The study by officials from Public Safety, the Correctional Service and the Parole Board of Canada concludes that achieving drug-free prisons is "an aspirational goal, just as is achieving drug-free societies."

"The fact that the goal has nowhere been realized, either in prisons or in society, does not undermine the importance of the goal but it does underscore that its achievement is an incremental process."

The heavily censored document and accompanying memo — both undated — were released to The Canadian Press under the federal law that allows people to seek copies of government records.

More than 80 per cent of federal prisoners have a substance abuse problem that requires intervention, according to the Correctional Service.

In the 2011 election campaign, the Conservatives said drug use among prisoners dramatically reduces their chances of rehabilitation. They promised that every federal inmate would be drug tested at least once a year, that prisoners with illicit substances would face appropriate additional charges, and that inmates who fail drug tests would be denied parole.

The study points out to the minister that drug testing in prison is commonplace but has serious limitations, and that prisoners are already subject to a range of sanctions if caught with drugs. It also says legislative changes would be needed to deny drug users parole, noting the independent parole board considers a variety of factors when weighing applications.

About half the inmate population takes a urine test each year. From 2006 through 2011, positive results accounted for 9.8 per cent to 12.7 per cent of the tests, though up to 12.6 per cent of inmates refused to participate.

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However, the study notes, not all substances can be detected, and a positive test cannot determine specifically when a drug was used, the exact dose taken or the degree of impairment caused. Moreover, some drugs clear the system in a matter of hours.

"Anecdotes suggest that when prisons institute more frequent drug testing, inmates simply switch to drugs that clear the body fastest" — which are often the most damaging ones, such as opiates, the document says.

A positive test or refusal can result in disciplinary action including reprimands, fines or segregation from other inmates. Like other citizens, prisoners can also be prosecuted under drug laws.

Parole Board members must review each case based on the risks associated with each individual inmate, the study notes.

"At the present time, legislation does not contain specific factors or mandatory grounds for the automatic denial of day or full parole, as this would be at odds with a risk-based framework for decision-making, as well as the (board's) discretion to make independent decisions."

Repeated positive tests or refusals to provide a sample may suggest an inmate's risk is not yet manageable on parole, the study says. On the other hand, a small number of infractions may indicate the prisoner is making significant progress in overcoming an addiction, and the use of appropriate conditions in the community will help things along.

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"And ultimately there will be some parole applicants who are occasional recreational users of drugs and alcohol and who are capable of living a stable, pro-social life upon release, much like the profile of any walk of life including public servants, parliamentarians, police, social workers, etc."

Nevertheless, the agencies developed policy options for the minister, all of which were blacked out from the document.

One insider said the government is considering which options to pursue, including the possibility of legislative change to ensure those who use drugs in prison do not receive parole.

The prison service has received tens of millions of dollars in recent years to help better keep drugs out of institutions and detect them once inside.

Since the increased focus on interdiction, drug seizures in prison have risen by 30 per cent, the government says.

Still, the study describes interdiction as "an overwhelming challenge."

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"The reality is that once an offender is incarcerated, the availability of illicit substances decreases, but does not diminish to zero."