Drain cleaners come in four major groups: acids, caustics, enzymatic cleaners and oxidizers. Acid drain cleaners tend to contain hydrochloric acid, while caustic cleaners are usually based on lye. Oxidizing cleaners can be based on a number of chemicals and are designed to break up organic materials. They are common additives to caustic cleaners. Enzymatic cleaners use biological methods to break down a block, and are relatively new to the market.
Chemical drain cleaners often produce heat, which may soften or warp plastic drain pipes. Corroded iron or steel pipes may also be damaged by heat and chemical reactions between the cleaner and the clog. These reactions sometimes force hot drain cleaner back out of the drain, where it can etch aluminum fixtures and crack porcelain ones.
Many chemical drain cleaners are inappropriate for use with septic systems. These chemicals can kill the bacteria that break down waste in the septic tank, causing it to work more slowly or stop entirely. Enzymatic drain cleaners are more appropriate for homes with a septic system, since these cleaners are less likely to damage the bacteria population in the septic tank. These newer cleaners do tend to work more slowly, including overnight.
In addition to possibly damaging your pipes and your fixtures, many chemical drain cleaners are a health and safety hazard. Acid or caustic drain cleaners may splatter on contact with water, and can cause chemical burns if they touch your skin. Inhaled fumes can hurt your throat or lungs. Handle these cleaners with care when you must use them, and turn to chemical cleaners only after you have tried mechanical means, such as a drain snake.