Toothpaste giant Crest is on the defensive after consumer complaints that one of its popular mouthwashes causes teeth to turn brown.

CTV News anchor Jina You started using Crest Pro Health and woke up one morning to find brown stains on her teeth.

"I looked in the mirror and all along my bottom teeth, it looked like I’d been chewing tobacco for 20 years, and it was very sudden. I just went 'oh my gosh, what caused this?'" You told CTV’s Steele on Your Side.

She’s not alone. Hundreds of angry consumers have taken to the internet and message boards claiming that the product also stained their teeth, leaving brown stains in between individual teeth.

“My teeth looked like they were rotting,” one user wrote on Amazon.com’s product review board.

“Quit using [it] and fortunately an expensive dental cleansing visit got the stains off… Don't use this stuff!” another user wrote.

Vancouver Dentist Shaireen Lalani says she’s not surprised by the problem and has seen similar anti-bacterial rinses cause dental staining before.

"It is tricky, some people have very crowded teeth and the stains can get in-between all the little cracks,” she said.

The parent company of Crest, Proctor and Gamble, told CTV News that the ingredient Cetyl Pyridinium Chloride is what causes some people’s teeth to turn brown.

The antiseptic agent is added to kill bacteria and ensure the mouthwash is alcohol-free, a major selling feature.

"Basically what happens is the product is killing bacteria that causes bad breath and in some individuals that bacteria as it's killed it can also turn brown and then stick to the teeth,” P&G spokesperson Victoria Maybee told CTV’s Lynda Steele in a telephone interview.

The product does have a small print warning on the back label that reads: “In some cases, antimicrobial rinses may cause surface staining to teeth," but consumers have complained the warning label is buried in the product information.

Maybee said Proctor and Gamble isn’t taking the situation lightly but the problem only affects a small percentage of users.

“It's something that I can only imagine like how upsetting that could be to a person,” she said.

The product does come with a satisfaction guarantee, and consumers can ask for a refund if they’re unhappy with the mouthwash.

Dentist Shaireen Lalani says the stains are not permanent and can be removed with professional cleaning.

In some cases, Crest has reimbursed consumers to have their teeth professionally cleaned.