Outraged parents are taking to social media to share photos of moldy Tommee Tippee sippy cups and calling for the manufacturer to investigate the potential health harm.

WGNTV.com reported that some parents are saying the mold on the popular cups can only be seen when opening their anti-leak spout.

Marie-Pier L’Hosite, of Montreal, Canada, is one of the people sharing photos of the cups, which belong to her friend Simon O’kanada. O’kanada told his friend that his son’s sickness persisted, and he couldn’t figure out why. When he opened the spout of his sippy cups, he discovered the mold. O'kanada didn't describe the symptoms his son was suffering.

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Now, photos of the cups, along with parents’ warnings about the mildew dangers, have been shared more than 45,000 times on Facebook, WGNTV.com reported.

Mon ami Simon O'kanada se demandait pourquoi son fils était toujours malade. Il a donc brisé l'anti fuite de son verre à...

Posted by Marie-Pier S. L'Hostie on Thursday, February 18, 2016

Tommee Tipee UK issued a statement to Tribune Broadcasting on Wednesday and said the mold issue is likely being caused by the use of non-recommended liquid, which includes thick formula milk, pulpy juice and warm liquids.

“We have reviewed the Facebook pages and are now in contact with [O'kanada] referred to in the original post. We are talking to him about his use of the cup as we always welcome parents’ feedback, comments and ideas,” the statement reads. “We’ve sold millions of Sippee cups and have had some very positive feedback. However, we know from visitor posts that a few people have not had a good experience with the valve part of the cup. For this we sincerely apologise and we are actively following up on any concerns raised.”

The statement also advised parents to clean sippy cups regularly. To avoid any potential harm, it indicated the company would be making a transparent valve available in the near future.