Development was stopped before the drug was even given a name and it is identified only as GW501516.
GSK was developing the drug to raise HDL, or good cholesterol, levels but dropped the project in 2006 when studies showed it caused a number of toxic effects, including a variety of cancers, in rats and mice.
The statement says it's not known what long-term use of the drug would do to people.
Health Canada says the drug is not approved for human use and was never made or distributed by GSK.
It notes that it isn't known whether the product being sold online as GW501516 is actually the drug or another compound being sold under that name.
The drug has been on the World Anti-Doping Agency's prohibited list for several years.
In March, the Montreal-based agency revealed some athletes had tested positive for use of the drug. And they took the unusual step of warning doping athletes that they need to steer clear of this drug for safety reasons.
"The side-effect of this chemical compound is so serious that WADA is taking the rare step of warning 'cheats' to ensure that there is complete awareness of the possible health risks to athletes who succumb to the temptation of using GW501516 for performance enhancement," the agency said.