Can drinking too much tap water say 12 glasses a day cause cancer? i mostly drink filtered water. i have 2-3 glasses around the time i go to bed.

Unlikely, however:: Pure water doesn't increase cancer risk. Unfortunately carcinogens can contaminate tap water but a good filter removes most of those. Unfortunately filters don't remove fluoride, which is often deliberately added and may increase risk of some cancers- see http://tinyurl.com/78mcgts Only 6 million people in the UK have fluoridated water- see http://tinyurl.com/z58x7kj If yours is then avoid it!