Does it hurt if my cat or dog drinks toilet water?

Pets may select toilets for quenching their thirst because the water is often quite cool.
Obviously, the pets' own bowl is the preferred source from an aesthetic perspective, but if the water in the bowl is clean, it is most likely that no harm will come of it.

Prevention tips:

* Provide fresh cold water at all times (may need to change it 2-4X daily)

* Make sure bowls are placed in easy-to-access spots—one may place one bowl on each story of a multi-storied home, especially when pets are old, young, or have mobility problems such as arthritis

* Add ice cubes to the water in the bowl or use an insulated bowl to help keep the water cold

* Close doors to bathrooms and keep toilet lids down

* One can even install childproof locks on the toilet lids for large, strong, or more determined pets.

Potential Problems:

If residue-type toilet bowl cleansers are left on the bowl (pucks) or placed in the toilet tank, some of these chemicals could be harmful to pets. If these are in use, it is important to close the lids and the bathroom doors securely. In homes with pets, it is inadvisable to use these products.
A problem can also occur if the pet gains access to the bathroom during routine weekly cleaning while the toilet bowl cleaner is soaking the bowl. Some of these products are very irritating/toxic and care should be taken to exclude pets from the bathroom during these cleaning routines.

If humans carry and shed certain bacterial diarrhea infections such as Salmonella or perhaps Giardia (the latter is a single-celled parasite), potential may exist for transmission of the agent to the pet if the bowl is not scrupulously clean. This should be considered a very rare scenario. The epidemiology of this route of transfer is not well studied or understood, but theoretically, fomites for agent transfer could potentially include soiled surfaces or water.