Saccharin is an artificial sweetener. The basic substance, benzoic sulfinide, has effectively no food energy and is much sweeter than sucrose, but has an unpleasant bitter or metallic aftertaste, especially at high concentrations. In countries where saccharin is allowed as a food additive, it is used to sweeten products such as drinks, candies, medicines, and toothpaste. Throughout the 1960s, various studies suggested that saccharin might be an animal carcinogen. Concern peaked in 1977, after the publication of a study indicating an increased rate of bladder cancer in rats fed large doses of saccharin. In that year, Canada banned saccharin while the United States Food and Drug Administration also proposed a ban. At the time, saccharin was the only artificial sweetener available in the U.S., and the proposed ban met with strong public opposition, especially among diabetics. Eventually, the U.S. Congress placed a moratorium on the ban, requiring instead that all saccharin-containing foods display a warning label indicating that saccharin may be a carcinogen. Many studies have since been performed on saccharin, some showing a correlation between saccharin consumption and increased frequency of cancer in rats (especially bladder cancer) and others finding no such correlation. No study has ever shown a clear causal relationship between saccharin consumption and health risks in humans at normal doses, though some studies have shown a correlation between consumption and cancer incidence. Some of the animal studies were procedurally flawed. According to tradegroup-operated saccharin.org, "Concerns over saccharin's safety were first raised twenty years ago after a flawed study that administered huge quantities of the sweetener to laboratory rats produced bladder tumors in rats. New and better scientific research has decisively shown that the earlier rat studies are not at all applicable to humans." The U.S. National Institute of Environmental Health Sciences came to the same conclusion in 2000, recommending that saccharin be removed from the list of known or suspected human carcinogens. The mechanism appears to be due the way that rats metabolize sodium, and bladder cancer which cannot be replicated in other mammals has also been observed with rat consumption of other sodium salts such as sodium citrate and bicarbonate. Saccharin has, however, also been linked to thyroid cancer in mice.  In 1991, after fourteen years, the FDA formally withdrew its 1977 proposal to ban the use of saccharin, and in 2000, the U.S. Congress repealed the law requiring saccharin products to carry health warning labels.