A clogged sink or tub is usually caused by an accumulation of organic material in the household plumbing such as hair, skin or makeup. As time goes on, the organic material builds up in the plumbing forming a bottleneck in the pipe causing the water to build up and not drain properly.
Drano is made of an acid composition that is strong enough to eat away at the organic blockage. The issue that often occurs is that when Drano is poured into the plumbing, the organic clog is released and frequently gets caught further down the plumbing creating a more serious or even a total blockage of the plumbing. The fact is, Drano and similar products are just temporary solutions. Clogged plumbing should be snaked or jet cleaned if you want it to last.

In addition to the further blockage of the plumbing, there is an ongoing debate on whether or not the strong acid composition of Drano and similar products damages the actual plumbing. PVC pipe does get brittle after several years, and Drano might accelerate the aging of the household plumbing. In most cases, a good old fashion drain cleaning by your local plumber is the best solution to a clogged drain.