Regarding health issues, it is unwise to ever drink tap water in Chengdu, or anywhere else in China.  Bottled water is cheap and available everywhere.  A hotel room will have bottled water available in the room.  Some people are fine with using the tap water for teeth brushing, but to be on the safe side, one can choose to use a bit of the bottled water instead.  Remember not to "gargle"  in the shower.  When purchasing a bottled beverage, be certain the seal is intact.  When purchasing a can of something to drink, clean the top of the can where one would drink from.   Raw fruits and vegetables can be problematic, since they are generally washed in the tap water.  Stick with fruit that can be peeled.  Cooked fruits and vegetables are fine.  There are several clincs that have "western" doctors on staff.  Out of country health insurance may not cover any illness or injury in China.  Check before going, otherwise expect to pay cash at time of visit.  If a prescription is needed, have it filled at the clinic being visited, to be certain of getting "real" medication.  

With regards to safety issues in Chengdu . . Chengdu is very safe for foreigners.    As with going anywhere, use common sense.  Exchange money only at a bank.  Use caution when crossing the street, the drivers here are notorious for disobeying traffic rules.  Red lights are but  a suggestion. It is not uncommon for cars to drive on the sidewalk or even directly into oncming traffic if the street is too crowded.   Taxi drivers will drive the wrong way down one way streets, drive into oncoming traffic, cut across 6 lanes of traffic if necessary, drive on sidewalks, and sometimes even make a road where there is not one.