More Canadian patients are joining a class-action lawsuit filed last year against the makers of an over-the-counter oral laxative, which has been linked to kidney damage in some patients who used it as a bowel preparation prior to having a colonoscopy.

The laxative, Fleet Phospho-soda, is believed to be safe when taken as a single-dose laxative, according to its directions.

However, using two doses of the product, as recommended, to cleanse the bowel prior to a colonoscopy may put users at higher risk of acute phosphate nephropathy, or kidney damage.

According to its manufacturer, Fleet Phospho-soda will no longer be manufactured or distributed in Canada, for commercial reasons.

Patrice, who asked that her last name not be used, said she did not learn until eight years after using the product that her kidneys were failing.

It was only after she had her kidney function tested when she was diagnosed with a sinus infection that she learned her kidneys were functioning at 35 per cent. Three months later, Patrice, 52, learned that her kidneys were functioning at 27 per cent.

"And (the doctor) said this is a problem, you're going to have to go on dialysis," Patrice, who has joined the lawsuit, told CTV News. "And at that point, I kind of fell apart."

Fleet Phospho-soda is one of a number of oral sodium phosphate (OSP) products, including the prescription-strength Visicol and OsmoPrep, that are used to clean the colon and have been linked to kidney problems.

The makers of Fleet Phospho-soda have been hit with a number of lawsuits in the United States by users who claim they suffered kidney damage after using two doses of the product prior to colonoscopies.

These lawsuits, as well as the Canadian suit, allege that the company knew, or ought to have known, that the product was unsafe at two doses.

According to Dr. Andrew House of the London Health Sciences Centre, some of those who have taken two doses of Fleet Phospho-soda may develop a specific type of damage, in which crystals form on the kidneys that can be seen on a biopsy. This can impede the kidney's ability to filter blood.

U.S. studies estimate that as many as 7,000 cases of kidney damage may be attributed to these products each year, House says.

However, many people will not immediately develop symptoms and will therefore not be aware of a problem.

And once a patient is diagnosed with kidney failure, he or she may never make the connection to use of the laxative.

"Our concern is that there are people out there that have been injured by it and don't know they were injured by this product," lawyer Matthew Baer, of Siskinds LLP, told CTV News.

Of all the colonoscopies that were done in Ontario over a six-year period, about 30,000 people developed kidney problems, Baer said.

However, it is unclear how many of these problems may be linked to bowel cleansers.

Lynda Quinton developed kidney damage after taking the Fleet Oral laxative before a colonoscopy. Two years later, she says she still can't work.

She hopes the lawsuit makes people more aware of how to use these products safely.

"I'm not saying don't use it," Quinton told CTV News. "But at least be aware and do the things that you have to do."

Late last year, the U.S. Food and Drug Administration issued so-called black box warnings, the strongest FDA warning, of the risks associated with Visicol and OsmoPrep and recalled the Fleet Oral laxative.

The FDA also warned consumers not to use over-the-counter products for bowel cleansing.

According to the FDA, consumers can minimize their risk of kidney damage by using them only under a doctor's supervision. Drinking a lot of fluids while taking the laxative may also help prevent against kidney damage.

That advice is too late for Patrice, who is facing major lifestyle changes as a result of her illness.

"Knowing that I'm going to be tied to a machine for X number of hours a week in order to live is very disheartening because I'm a very spontaneous person," Patrice said. "I like to just get up and go when I feel like it, and I won't be able to do that now. Doing anything is going to require some planning -- I'm going to have to plan my life around this machine."

With a report from CTV medical correspondent Avis Favaro and senior producer Elizabeth St. Philip