Amalgam fillings fail less often than composite. FDA's caution is about specific groups.

Amalgam fillings outperform composite resin on durability by a wide margin in the randomised evidence, and the safety case against them has not been demonstrated in the general population. A Cochrane review of trials in permanent posterior teeth found composite restorations carried almost double the risk of failure (risk ratio 1.89, 95% confidence interval 1.52 to 2.35) and more than double the risk of secondary caries (RR 2.14, 95% CI 1.67 to 2.74), both on low-certainty evidence [s2]. The FDA's position is that available evidence does not show mercury exposure from dental amalgam leads to adverse health effects in the general population, while identifying specific groups who may face greater risk [s1].

What amalgam is, and what the FDA says about it

Dental amalgam is a mixture of liquid elemental mercury and a powdered alloy of silver, tin and copper; approximately half of it — 50% by weight — is elemental mercury [s1]. It releases low levels of mercury in the form of a vapour that can be inhaled and absorbed by the lungs [s1]. Exposure to high levels of mercury vapour, which may occur in some occupational settings, has been associated with adverse effects in the brain and the kidney [s1].

The FDA lists amalgam's benefits plainly: strong and long-lasting, so less likely to break than some other filling types; useful in patients at high risk of decay, for large tooth preparations, and where moisture makes it difficult for resin materials to bond; the least expensive filling material; and in use for more than 150 years in hundreds of millions of patients worldwide [s1].

Its risk framing is specific rather than general. Developing neurological systems in fetuses and young children may be more sensitive to the neurotoxic effects of mercury vapour, and there is very limited to no clinical data on long-term health outcomes in pregnant women and their developing fetuses, and in children under six including breastfed infants [s1]. The agency identifies groups who may be more susceptible: pregnant women and their developing fetuses; women planning pregnancy; nursing women and their newborns and infants; children, especially those younger than six; people with pre-existing neurological disease; people with impaired kidney function; and people with known heightened sensitivity or allergy to mercury or to the silver, copper or tin in amalgam [s1].

Two clarifications from the same source cut against widespread claims. The mercury in amalgam is elemental mercury, releasing vapour absorbed mainly by the lungs, while the mercury in fish is methylmercury, absorbed mainly through the digestive tract — the body handles them differently and tolerates them differently, though the FDA notes that some recent studies show inorganic and organic mercury can transform into each other in the body [s1]. And while mercury from amalgam does bioaccumulate, studies have not shown that the resulting levels produce detectable damage to target organs [s1].

The removal question has a clear answer

If fillings are in good condition and there is no decay beneath them, the FDA does not recommend having amalgam fillings removed or replaced [s1]. Removing intact fillings causes unnecessary loss of healthy tooth structure and exposes the person to a temporary increase in mercury vapour released during the removal process [s1]. That guidance applies to everyone, including the sensitive groups listed above: intact amalgam fillings should not be removed for the purpose of preventing any disease or health condition unless considered medically necessary by a health care professional [s1].

That is the single most practically useful line in the FDA's material, and it is the opposite of what a market in amalgam removal is built on.

What the trials found on both durability and harm

Cochrane's review included eight randomised controlled trials — two parallel-group and six split-mouth designs — all judged at high risk of bias because of the impossibility of blinding and because of unit-of-analysis problems [s2]. The primary meta-analyses combined the two parallel-group trials, covering 1,645 composite restorations and 1,365 amalgam restorations in 921 children [s2].

Composite resin restorations had almost double the risk of failure (RR 1.89, 95% CI 1.52 to 2.35; P < 0.001) and a much higher risk of secondary caries (RR 2.14, 95% CI 1.67 to 2.74; P < 0.001), on low-certainty evidence [s2]. They were not more likely to fracture (RR 0.87, 95% CI 0.46 to 1.64; P = 0.66) [s2]. Six split-mouth trials, considered separately because poor reporting and methodological variability compromised their reliability, produced findings consistent with the parallel-group results [s2].

Three trials investigated harms directly [s2]. Two reported higher urinary mercury levels among children with amalgam restorations, but at levels lower than what is known to be toxic [s2]. Some differences between groups were observed on certain measures of renal, neuropsychological and psychosocial function, physical development and postoperative sensitivity — and the review authors attribute those to the sheer number of comparisons performed, which makes false positives likely [s2]. There was no evidence of differences in neurological symptoms, immune function or urinary porphyrin excretion [s2]. The certainty of all of that harms evidence is very low, with most outcomes reported in only one trial [s2].

Why amalgam is being phased down anyway

The reason amalgam use is declining is not primarily a finding about patient safety. The Minamata Convention on Mercury, an international treaty aiming to protect human health and the environment from anthropogenic emissions and releases of mercury, entered into force in August 2017 and had been ratified by 127 governments as of February 2021 [s2]. Ratification commits a country to adopting at least two of nine proposed measures to phase down mercury use, including amalgam in dentistry [s2].

Cochrane's own conclusion sits awkwardly alongside that trajectory, and says so: the review supports the utility of amalgam restorations, and the results may be particularly useful in parts of the world where amalgam remains the material of choice [s2]. The authors also note that composite resin materials have improved substantially since the trials informing the primary analyses were conducted [s2] — a caveat that cuts the other way, and one worth holding onto, since the durability figures above come from older materials.

This article summarises regulatory information and published trial results and is not dental advice. The choice of restorative material for any individual tooth depends on its size, position, moisture control and the patient's circumstances, and is a decision for a dentist and patient together [s2].