Since the 1970s, there has been controversy in the dental profession regarding the safety of mercury as a filling material. Some dentists contend that no matter what the amount used in dental fillings, the mercury is toxic. At the same time, the American Dental Association (ADA) asserts that only a small percentage of individuals exhibit allergies to mercury, and that once mercury is mixed and hardened as a filling material, the percentage of individuals who are sensitive to it becomes even smaller.

While the ADA has maintained that mercury in amalgam is stable, various researchers have demonstrated that it is not. In 1970, dentist Wallace Johnson and others published the results of their research in Journal of the American Dental Association. The report indicated that "mercury, an essential element of dental amalgams, is not a stable material but one that vaporizes at ordinary temperatures." In 1985, the Journal of Dental Research published a report by dentists Murry Vimy and F.L. Lorscheider, who also claimed that mercury vapor is released from amalgam fillings. These researchers further stated that the vapor is then distributed throughout the body via the respiratory system, but accumulates mostly in the brain, kidneys, and gastrointestinal tract. Some of the symptoms associated with mercury toxicity include a metallic taste in the mouth, severe headaches, dizziness, fatigue, weakness, depression, hair loss, memory loss, coma, and death. Furthermore, a natural balance of microorganisms in the body is upset by excess mercury in the gastrointestinal tract, and this imbalance may lead to candida.

In addition, the previously mentioned researchers have reported that when the metal comes in contact with saliva, there is a "battery effect" in which currents are generated, causing mercury molecules from the surface of the fillings to be released into the tissues.

Another negative report was published in the Journal of Prosthetic Dentistry in 1983. David Eggleston, a dentist in private practice and a researcher and professor at the University of Southern California School of Dentistry, indicated that when amalgam and nickel-based fillings are present, there is a reduction in the T-lymphocytes of the immune system. Lymphocytes are white blood cells that are the core of the body's immune system. Lymphocytes that circulate through the thymus gland are called T-lymphocytes (thymocytes). They recognize foreign invaders in the body and form antibodies against them. Eggleston concluded that the introduction of mercury into the body might weaken the immune system by reducing the number of T-lymphocytes.