Ever since childhood, as an easy solution to prevent tooth decay. Even most toothpaste brands include some amount of flouride,

so do most municipal water supplies.  According to a Harvard study in 2012, children who drank water with fluoride ended up with lower intelligence scores than children who didn't.  Researchers also linked fluoride drinking by pregnant mothers with lower IQ score in their children.

Unfortunately, that's not the only risk...Fluoride consumption is also linked to thyroid disease and cancer.

Water fluoridation - the addition of fluoride to the public water supply - has been terminated in many countries - including Germany, Sweden, and Japan - on suspicion of health harms. Cavity rates are no different. And yet, the U.S. continues to dump this dangerous chemical into your water.

To limit your exposure, avoid toothpaste with fluoride, limit the amount of tap water you drink.  Look for bottled water without flouride or install a reverse-osmosis water filter for drinking and cooking.  See here for a study of 24 bottled water score on various contaminants. 

Source:

http://secure.stansberryresearch.com/secure/rem/issues/pdf/201305REM_issue.pdf
 http://articles.mercola.com/sites/articles/archive/2012/07/14/water-fluoridation-and-healthy-teeth.aspx