Last week a dad posted a warning to let parents know about the mould he found in his daughter's Sistema Twist'n'Sip drink bottle, saying it was "hoarding bacteria".
Now sippy cups are in the spotlight as parents around the world are noticing their children's Tommee Tippee cups aren't as clean as they'd like to think.
One mum, Penny Powell, from Montreal, Canada, shared photos of different cups on Facebook, with many showing what appears to be mould in the anti-spill guard top, the section that holds the mouthpiece together.
"My friend wondered why his son was still sick so he broke open the anti-spill guard of his Tommee Tippee cup and discovered mould inside the anti-spill top that you can't see except if you break it open," she wrote (translated from French).
"He has washed it with his hands and in the dishwasher and the mildew has stayed anyway."
The original post has now been shared almost 100,000 times.
Thousands of parents have commented, with many sharing their own photos of mould in their own cups.
"WARNING: All the people who have been using anti-spill cups for some time THROW THEM OUT! Open your cups, remove the anti-spill guard and break it into two. Mold settles in the cap and makes children sick!! … Neither the dishwasher or washing by hand removes it!!" one mum wrote.
Tommee Tippee representatives have responded to the posts, saying they "are actively working on the issue".
"We learned, as a result of posts, that some people were not satisfied with the result of the sippy valves used in our sippy cups: We apologize for the inconvenience," they wrote on Facebook.
In a statement to Buzzfeed, a Tommee Tippee spokesperson explained that the cups are fine when used in their recommended way – with cold liquids only.
"The results support that when used with recommended liquids (cold, light fluids including water and non-pulp juices) and cleaned in line with instructions, there are no problems. Difficulties have arisen though when liquids that are not recommended for use in the cups have been used, like thick formula milk, pulpy juice and warm liquids.
They also shared instructions for proper cleaning, adding, "We also recommend that cups are not left for long periods before being cleaned."