Fluoride helps protect your teeth from cavities, which is why your dentist will give you a minute’s worth of fluoride treatment after your teeth cleaning, or why toothpastes often contain the chemical. It’s also why the U.S. government began putting low amounts of fluoride into drinking water, making it a national policy by 1951 in order to decrease the prevalence of cavities in the country.

But despite the fact that nearly 400 million people worldwide receive artificially fluoridated water (mainly in the U.S., some parts of South America, and Australia), the process has remained controversial. Now, a new study carried out by Professor Stephen Peckham at the Centre for Health Service Studies at the University of Kent in the UK claims that fluoride in drinking water is actually far more harmful than we thought, and it can lead to an increase in underactive thyroid — which is linked to depression, obesity, and fatigue.

The study found that in high-fluoride areas like the West Midlands and the North East of England, there was a spike in underactive thyroid cases. Underactive thyroid cases were 30 percent more likely in the areas of highest fluoridation, or over 0.3 mg/l.

“I think it is concerning for people living in those areas,” Peckham said, according to The Telegraph. “The difference between the West Midlands, which fluoridates, and Manchester, which doesn’t was particularly striking. There were nearly double the number of cases in Manchester.”

Peckham argues that water fluoridation should be replaced with other forms of dental care. “Underactive thyroid is a particularly nasty thing to have and it can lead to other long term health problems,” he continued. “I do think councils need to think again about putting fluoride in the water. There are far safer ways to improve dental care.”

Water Fluoridation: The Controversy

Opposition to water fluoridation has existed since it began in the 1940s. Many people have believed it's a form of compulsory mass medication, and even went so far as to claim that it was part of a communist plan to undermine public health during the 1950s and 60s. Though no adverse health effects have been found at the current level of fluoride in the water — aside from dental fluorosis, which is largely harmless — people continue to question and repute the notion of government-forced water fluoridation. One main argument is that it could be a violation of ethical or legal rules that prevent forced medical treatment without a person’s informed consent.

One of those well-known public opponents seems to be Peckham himself. In another study carried out by Peckham and co-autor Niyi Awofeso, published in 2014, he argued that “available evidence suggests that fluoride has a potential to cause major adverse human health problems, while having only a modest dental caries prevention effect.” Another study identified fluoride as a chemical potentially linked to mental disorders like ADHD and autism. And yet other research has shown that very high levels of fluoride could cause neurotoxicity in both adults and children, which might have an effect on memory and learning.

But the low levels of fluoride placed in drinking water has been deemed safe by governmental health agencies after decades of research. And other research on whether water fluoridation increases a person’s risk of cancer has proved rather inconclusive: some studies have shown it may increase the chance of developing osteosarcoma, a rare type of bone cancer, while others have shown no link between higher levels of fluoridation and more incidents of osteosarcoma.

The Environmental Protection Agency (EPA) states that yes, fluoride might lead to some health problems if “present in public or private water supplies in amounts greater than the drinking water standard set by EPA.” Some of these include an increased risk of bone fractures and cosmetic effects on teeth. But the water that’s entering your home will likely be lower than such levels.

It’s hard to say whether fluoride in drinking water causes an underactive thyroid. Some experts note that the recent study has its flaws: "It is quite possible that the observed association is a consequence of other ways in which the areas with higher fluoride differ from the rest of the country," Professor David Coggon of the University of Southampton, told The Telegraph. "There are substantially more rigorous epidemiological methods by which the research team could have tested their idea."