Having more than eight fillings could raise blood mercury levels two-and-a-half times, increasing risks of brain, heart and kidney disease, a study shows.
They are made of a mixture of metals which also include silver and tin.
Now for the first time scientists have found a link between tooth fillings and mercury exposure - which can lead to severe illnesses.
In Britain more than one million children under the age of five already has at least two fillings.
Brits have a reputation for bad teeth with the average adult having seven fillings - compared to three in the US.
Researcher Dr Lei Yin, of Georgia University, said: "Tooth decay is one of the most prevalent chronic diseases.
"I think a majority of people have experienced dental fillings but the kind of materials the dentist uses is not something that is really discussed."
Dr Yin and colleagues analysed data from almost 15,000 people to identify a link between dental fillings and potential mercury poisoning for the first time.
Fillings are put together using mercury, silver, tin and copper to form a liquid called dental amalgam which then hardens.
Mercury makes up about 50 percent of the compound and is used to bind the metals and provide a strong, hard, durable product.
But it contributes significantly to prolonged mercury levels in the body, according to the findings published in Ecotoxicology and Environmental Safety.
Mercury exposure from fillings is not a new concern but previous studies were inconsistent and limited, say the researchers.
Professor John Yu said: "This study is trying to provide the most accurate levels of exposure which will form the scientific basis to make future risk assessment."
He said the study was the first to also control for age, education, ethnicity, race, gender, smoking and seafood consumption - a known contributor to mercury levels in the body.
Further analysis of exposure to specific forms found a significant increase in methyl mercury - the most toxic form - related to dental fillings.
Prof Yu said this suggests the human gut bacteria living in the intestines may transform different types.
Dental amalgam has been the go-to dental filling material form over 150 years because it's affordable and long-lasting.
But the heavy metal is known to be toxic at high levels causing brain, heart, kidney, lung and immune system damage.
Latest research suggests methyl mercury could cause damage even at low levels.
Prof Yu said: "As toxicologists we know mercury is poison but it all depends on the dose. So if you have one dental filling maybe it's OK.
"But if you have more than eight dental fillings the potential risk for adverse effect is higher."
People with numerous dental fillings who are also exposed to mercury from other sources - such as seafood or work environments - are most at risk.
Individuals with more than eight fillings had about 150 per cent more mercury in their blood than those with none.
According to its website the US Food and Drug Administration considers dental amalgam fillings safe for adults.
But it says: "Pregnant women and parents with children under six who are concerned about the absence of clinical data as to long-term health outcomes should talk to their dentist."
The study also looked at dental composite resins - a mercury-free alternative for fillings that can release small amounts of bisphenol A (BPA).
It found no evidence of the hormone disrupting chemical in urine samples but the researchers said further studies are required.
Dr Yin said: "It's important for doctors and patients to be informed in their selections.
"We now have an excellent starting point to evaluate the potential risk of dental material on human health."