The performance of some drugs can be affected by the contents of some drinks. For example, the absorption of vitamin C into the body can be affected by caffeine, so some vitamin supplements should not be swallowed with tea or coffee. Similarly, grapefruit juice is a no-no with certail types of drug. I dont know exactly why, but its certainly to do with reactions between the drug and the contents of the drink. With so much of the body already composed of water, the performance of drugs taken by mouth are unlikely to be affected by being swallowed with water.