Electronic cigarettes have been promoted as a safe alternative to tobacco, but health agencies in Canada and the U.S. are flexing some regulatory muscle against those who distribute and sell them.

A researcher at the University of Alberta's Department of Public Health Sciences takes a drag off an electronic cigarette as part of a study at the university. (Trevor Brine/CBC)The U.S. Food and Drug Administration issued warning letters to five distributors, Thursday, citing unsubstantiated claims and poor manufacturing practices.

The FDA is also following Health Canada's lead in regulating electronic cigarettes as drugs.

The warning letters delivered to U.S. distributors, among other things, takes issue with claims that e-cigarettes can help smokers quit using tobacco. The FDA says that claim has not been proven.

One distributor, E-Cig Technology was also cited for marketing drugs in unapproved liquid forms, which are added to refill cartridges, vaporized and inhaled. Among the drugs being consumed through e-cigarettes are the erectile dysfunction drug Tadalafil, and Rimonabant, a weight loss medication.

The FDA also cited Johnson Creek Enterprises — which markets smoke juice, a liquid solution used to refill cartridges in e-cigarettes — with significant deficiencies in its manufacturing process, including failure to establish quality control and testing procedures.

E-cigarettes have been available to Canadians through online sales and some retailers have been selling them. However, Health Canada says they are not approved for sale in Canada.

Last year Canada's public health agency issued a warning letter stating the sale and advertising of electronic cigarettes in Canada is a violation of the Food and Drugs Act.