In 1977, the U.S. Congress requested that animal studies be conducted to determine if fluoride can cause cancer. The result of the Congressional request was an extensive animal study conducted in the 1980s by the National Toxicology Program (NTP) and published in 1990.

The principal finding of NTP's study was a dose-dependent increase in osteosarcoma (bone cancer) among the fluoride-treated male rats.

However, despite the fact that 1) the cancer occurred in the target organ (bone) for fluoride accumulation, that 2) the increase in bone cancer was statistically-significant, that 3) the doses of fluoride were low for an animal cancer study, and that 4) NTP acknowledged it is "biologically plausible" that fluoride could induce bone cancer, the NTP ruled that the study only provided "equivocal evidence" that fluoride was the cause of the cancer.

According to a report in Chemical & Engineering News: "A number of government officials who asked not to be identified also have told C&EN that they have concerns about the conclusions of the NTP study. They, too, believe that fluoride should have been placed in the "some evidence" category, in part because osteosarcoma is a very rare form of cancer in rodents."

In 2002, the World Health Organization (WHO) advised scientists to take NTP's finding seriously. According to the WHO: "Such a (dose-dependent) trend associated with the occurrence of a rare tumour in the tissue in which fluoride is known to accumulate cannot be casually dismissed."

In addition to increased bone cancer, the NTP study also found increases in rare liver cancers, oral cavity cancers and thyroid cancers among the fluoride-treated rats. The NTP ruled, however, that the cancers were not related to the fluoride treatment - despite reaching "statistical significance" in some of NTP's analyses.

Fluoride Linked to Bone Cancer in Fed Study - Medical Tribune December 28, 1989

Don't Drink the Water? - Newsweek February 5, 1990

Caries preventative already has one rap against it - Medical Tribune February 22, 1990

Rat Study Reignites Dispute On Fluoride - New York Times March 13, 1990

Weak Link on Fluoride and Cancer Is Backed - New York Times April 27, 1990

ACSH Considers Taking Legal Action Against Attempts to Reclassify Fluoride - Food Chemical News April 30, 1990

The Risks of Fluoride: The Long Awaited Verdict Newsweek May 7, 1990

Fluoride bioassay study under scrutiny - Chemical & Engineering News September 17, 1990

More about fluoride - The Lancet September 22, 1990

EPA Ordered to Reinstate Whistleblower - The Associated Press December 18, 1992

Reich Orders EPA to Reinstate Scientist - National Whistleblower Center February 10, 1994

Scientist Who Spoke Out on Fluoride Ordered Reinstated to Job - The Associated Press February 11, 1994

Interviews with EPA Scientists - NTP Fluoride/Cancer Bioassay: (back to top)

"Osteosarcomas of the bone were observed in 3/80 (4%) high-dose and in 1/50 (2%) mid-dose male rats. An additional osteosarcoma, which was determined to be of subcutaneous origin, was observed in a fourth high-dose rat. No osteosarcomas were seen in controls or in male rats receiving 25 ppm. The neoplasms were clearly malignant (one metastasized to the lung) and there was complete agreement concerning the diagnoses at both the Quality Assessment and the Pathology Working Group stages of histopathology review...

Osteosarcomas (in bone or extraskeletal) are not commonly observed in control male rats in NTP studies. The historical incidence in control male rats from dosed feed or water studies is 10/2,106 (0.47%)...

The four osteosarcomas of bone (one in the mid-dose and three in the high-dose groups) in the current studies occurred with a statistically significant dose-response trend by the logistic regression test (P=0.027); the pairwise comparison of the incidence in the high-dose group versus that in controls was no statistically significant (P=0.099). The statistical significance of the trend test is increased (P=0.010) when the subcutaneous osteosarcoma in the fourth high-dose rat is included in the incidence, but the pairwise comparison remains not significant (P=0.057). The incidence of bone osteosarcomas of 3/80 and the incidence of all osteosarcomas of 4/80 in the high-dose male rats are both significantly greater than the rate of 0.6% for osteosarcomas and osteomas at all sites in control male rats in the historical database...

To summarize these considerations, a small number of osteosarcomas occurred in mid- and high-dose male rats. These neoplasms occurred with a significant dose response trend, but at a rate wtihin the upper range of incidences previously seen in control male rats in NTP studies. Three of the tumors arose in the vertebra, a site not commonly associated with chemically induced osteosarcomas. Bone is known to accumulate fluoride, and fluoride has been shown to be genotoxic to some mammalian cells in culture. No osteosarcomas were seen in female rats, and several osteosarcomas seen in mice occurred with an incidence that did not suggest a relationship with sodium fluoride exposure. Taken together, the current findings are inconclusive, but are weakly supportive of an association between sodium fluoride administration and the occurrence of osteosarcomas in male rats."
SOURCE: National Toxicology Program [NTP] (1990). Toxicology and Carcinogenesis Studies of Sodium Fluoride in F344/N Rats and B6C3f1 Mice. Technical report Series No. 393. NIH Publ. No 91-2848. National Institute of Environmental Health Sciences, Research Triangle Park, N.C. p. 71-73.

"Such a (dose-dependent) trend associated with the occurrence of a rare tumour in the tissue in which fluoride is known to accumulate cannot be casually dismissed."
SOURCE: World Health Organization. (2002). Environmental Health Criteria 227: FLUORIDES. World Health Organization, Geneva.

"the level of fluoride the low- and mid-dose animals had in their drinking water was within an order of magnitude of what humans are exposed to when drinking water containing the EPA-established maximum level of 4 ppm fluoride. This is almost unheard of in animal bioassays. Usually, animal exposure is four to six orders of magnitude more than what humans receive."
SOURCE: Hileman B. (1990). Fluoride bioassay study under scrutiny. Chemical & Engineering News September 17.

"it is important to note that the dose range is not, as is sometimes the case, orders of magnitude higher than that encountered in human population, nor is the body burden expressed as concentrations in bone orders of magnitude higher than that found in human populations also ingesting fluoride."
SOURCE: Silbergeld E. (1990). Peer Review of Draft Technical Report of Long-Term Toxicology and Carcinogenesis Studies and Toxicity Study, Sodium Fluoride; Research Triangle Park, North Carolina, Thursday, April 26, 1990. p. 62-63.

"the difference between the animal study and the human exposures is not nearly as great as typical with synthetic chemicals."
SOURCE: Gold L. (1990). Peer Review of Draft Technical Report of Long-Term Toxicology and Carcinogenesis Studies and Toxicity Study, Sodium Fluoride; Research Triangle Park, North Carolina, Thursday, April 26, 1990. p. 71.

"I think it's important to realize that even though the water concentrations were higher than what we see, or what humans are exposed to, the bone concentrations were not."
SOURCE: Zeise L. (1990). Peer Review of Draft Technical Report of Long-Term Toxicology and Carcinogenesis Studies and Toxicity Study, Sodium Fluoride; Research Triangle Park, North Carolina, Thursday, April 26, 1990. p. 79.

Substance	Daily Dose (mg/kg/day)	Maximum Contaminant Level (MCL) (mg/litre)
Fluoride	7.9	4
Vinyl chloride	1.7	0.002
Carbon tetrachloride	47	0.005
Benzene	50	0.005
Chloroform	160	0.100
Tetrachlorethylene	386	0.005
Red dye #3	4000	none
Data Compiled by Dr. William Marcus, EPA, March 1990.

Non-Bone Tumors in NTP Study - Fluoride & Liver Cancer: (back to top)

For information on the liver tumors in the NTP study, click here

Non-Bone Tumors in NTP Study - Fluoride & Oral Cancer: (back to top)

"A second potential target site for sodium fluoride when given in drinking water is the upper digestive tract and oral cavity. Squamous cell neoplasms of the oral mucosa (tongue, palate, or gingiva) occurred with marginally increased incidences in dosed males and female rats over the rates in controls. The increased incidences of these neoplasms were not statistically significant when compared with the incidences in concurrent controls; however, the incidences in the high-dose groups were significantly higher than the incidences observed in historical control animals (0.7% male rats; 0.6% female rats).

As with lesions of the bone, a direct comparison with the historical rates for oral cavity neoplasms is not completely accurate because of the increased attention given to the oral cavity and teeth in the sodium fluoride studies compared to previous NTP studies. Rates for oral cavity neoplasms similar to those observed in high-dose male and female rats in the sodium fluoride studies (4%) have been observed twice for males and once for females in the historical control database of 42 dosed feed or water studies. Neoplasms of the oral cavity were observed in control male and female rats in the current studies; one was observed in an age-matched control male rat and one occurred in a control female rat in the main study.

An argument could be made for combining the male and female rat studies for analysis of oral cavity neoplasms because a marginal increase occurred in both groups. An analysis for significance of the combined P values for the logistic regression trend tests for males and female rats resulted in a nonsignificant P value of 0.065.

In contrast to osteosarcomas, for which there are no recognized benign or preneoplastic counterparts (Litvinov and Soloviev, 1973), squamous cell hyperplasias of the oral cavity are considered preneoplastic precursor lesions of squammous cell neoplasms of the oral cavity (Brown and Hardisty, 1990). Squamous cell hyperplasia occurred in no more than one animal in any of the dosed or control groups in the current studies. Thus, based on the absence of statistical significance versus the concurrent controls, the occurrence of these tumors in control animals, and the lack of a dose-related increase in non-neoplastic precursor lesions, it is concluded that there is insufficient evidence to relate tumors of the oral cavity with administration of sodium fluoride to male or female rats. Glattre and Wiese (1979) reported an association between a decrease in human mortality due to oral cavity neoplasia and increasing fluoride content in water over the range of 0 to 0.5 ppm."
SOURCE: National Toxicology Program [NTP] (1990). Toxicology and Carcinogenesis Studies of Sodium Fluoride in F344/N Rats and B6C3f1 Mice. Technical report Series No. 393. NIH Publ. No 91-2848. National Institute of Environmental Health Sciences, Research Triangle Park, N.C. p. 73-74.

Non-Bone Tumors in NTP Study - Fluoride & Thyroid Cancer: (back to top)

"Follicular cell neoplasms of the thyroid gland appeared with a marginally increased incidence in high-dose male rats compared with controls. This increase is not statistically significant compared with controls unless control animals from both interim groups (27 and 66 weeks) and the age-matched controls are pooled with the main study control group. If this is done, the logistic regression P value for the trend is 0.027. Thyroid follicular cell neoplasms typically occur with an incidence of 1.2% in historical control animals. Incidences of 6% have previously been observed in untreated control groups for gavage studies. The incidence of these neoplasms in the high-dose groups was 5/90 (5.5%; includes 10 animals from the 66-week interim sacrifice, one of which had a thyroid follicular cell carcinoma). Three of these tumors were adenomas. The incidence of carcinomas did no differ across the dosed groups and the incidence of follicular cell hyperplasia was not increased. No increase in the incidence of these tumors occurred in female rats. Based on these considerations, follicular cell neoplasms of the thyroid are not considered related to sodium fluoride administration."
SOURCE: National Toxicology Program [NTP] (1990). Toxicology and Carcinogenesis Studies of Sodium Fluoride in F344/N Rats and B6C3f1 Mice. Technical report Series No. 393. NIH Publ. No 91-2848. National Institute of Environmental Health Sciences, Research Triangle Park, N.C. p. 74.