The discovery of a 16-year-old girl at a Halloween party in Quebec who was found naked with traces of the date-rape drug GHB in her bloodstream has renewed concerns about the prevalence of these incapacitating substances.

The incident occurred at the Spookland Halloween party at Mont Cascades Ski Resort & Water Park in Cantley, Que., just north of Ottawa, on Friday. In total, six girls and three boys were taken to hospital.

While police continue to investigate the incident, one partygoer told CBC News that someone was handing out pills on her bus as she was heading to the event.

What are date-rape drugs?

Date rape drugs are substances that, according to the Vienna-based International Narcotics Control Board, are used "to weaken the resistance of individuals, for example to exploit their property or body with their apparent consent, without them having the slightest recollection afterwards of what happened."

The most common ones are GHB (gamma-hydroxybutyrate), rohypnol (flunitrazepam) and ketamine.

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University of Saskatchewan student Michelle MacDonald told CBC that she believes she was given a date-rape drug at a Saskatoon bar in 2013. (Don Somers, CBC)

GHB and rohypnol are central nervous system depressants that are odourless and flavourless, which means they can be added to an alcoholic drink without being detected.

Another common drug is ketamine, which is used by veterinarians to anesthetize animals.

The use of date-rape drugs is not a new phenomenon.

According to a 2009 report by the INCB, benzodiazepines such as flunitrazepam have a history of misuse. Flunitrazepam was first included in the Convention of Psychotropic Substances, a UN treaty to control psychoactive drugs, in 1984.

What are their effects?

The effects of all these substance can include dizziness, memory loss, confusion and loss of consciousness.

The RCMP web site says that the effects of GHB "can be felt as soon as 10 to 20 minutes after ingesting it, and can last up to four hours."

How easy are they to detect?

According to a report prepared by the University of Alberta, the most ubiquitous date-rape drugs can be difficult to detect because the human body processes them fairly quickly.

For example, GHB can be untraceable in the urine after only 12 hours, whereas rohypnol will be traceable in the urine and blood for up to 72 hours.

How are they classified under the law?

GHB and rohypnol are illegal under Canada's Controlled Drugs and Substances Act.

According to the RCMP, the drugs that are in circulation have been manufactured in illegal laboratories.

How commonly are these drugs used?

A 2009 study by the Canadian Medical Journal found that 21 per cent of reported sexual assaults involved date-rape drugs, up from 12 per cent in 2003.

The study screened 882 women over the age of 16 who had been sexually assaulted. 21 per cent, or one in five, believed they were intentionally drugged prior to being sexually assaulted.

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Edmonton Police display some of the 106 litres of GHB seized in a drug bust in 2013. (CBC)

However, of the substances found in the women's systems, "very few of those were what people classically refer to as date-rape drugs," says Janice Du Mont, an associate professor at the Dalla Lana School of Public Health at the University of Toronto and the lead author of the study.

"We found only one case of GHB, no rohypnol, a couple of cases of ketamine."

Du Mont acknowledges it can be hard to pinpoint how widely date-rape drugs are used, precisely because GHB and rohypnol are both untraceable after 72 hours.

Even so, Du Mont says "the big drugs" that came up in the 2009 study sample were street drugs such as marijuana, cocaine, amphetamines and analgesics such as codeine and morphine.

She says that there is "a lot of hype" about so-called date-rape drugs, but she says the amount of media coverage of them might be misleading.

"It's not that women and some men aren't being surreptitiously drugged and sexually assaulted, but maybe the question might be the common drugs that are being used."

What is the most common date-rape drug?

Du Mont says that, generally speaking, "alcohol is implicated in 50 per cent or more of sexual assaults, either the victim or the assailant or both."

This points to the fact that "alcohol is the number one-date rape drug," says Lenore Lukasik, chair of the Ontario Coalition of Rape Crisis Centres.

"You're at risk of experiencing sexual violence if someone slips something in your drink, absolutely, but there are also folks that get people drunk, and that's how they then perpetrate the violence," says Lukasik.

Is it possible to charge someone with the use of a date-rape drug?

A key determinant in bringing a charge of sexual assault against someone is a lack of consent on the part of the victim. And "if somebody's impaired by alcohol or any other substance, they cannot consent to a sexual act," says Eugene Oscapella, a lawyer and professor of drug policy at the University of Ottawa.

That said, charging someone with "drug-facilitated sexual assault" is challenging because it is difficult to prove that a person intended to incapacitate another individual, says Oscapella.

This is especially tricky given how quickly date-rape drugs can leave the bloodstream.

"How do you go about showing that this person gave this substance, knowing what it was, and used it for this purpose, so that they could facilitate the act?"

Oscapella says if police found a person who committed sexual assault in possession of GHB or rohypnol, they could make a reasonable case that it was premeditated.