Pouring bleach down the drain is generally not recommended, as bleach is a corrosive substance and can pose a health and safety risk if it combines with other common household products. Chlorine bleach is highly concentrated but poses few environmental and health hazards when used according to the manufacturer's instructions, say authors at New Hampshire Public Radio (NHPR).
The biggest threat to human health from dumping chlorine down the drain comes from its combination with other substances, particularly ammonia. A safer alternative to bleach for unclogging drains is boiling water, say authors at NHPR. People should slowly pour a tea kettle's worth of boiling water down their drains, which eliminates grease and grime just as effectively as bleach.