It is a very volatile chemical, it is very damaging to your eyes, respiratory tract and skin. It is a strong corrosive. It will irritate or burn the skin, eyes and respiratory tract. It may cause pulmonary edema or vomiting and coma if ingested. Most formulas are designed to over power the stain itself, they accomplish the task but not without using highly toxic substances.
Most products contain chlorine in a dry form that is highly concentrated.
Most drain cleaners contain lye, hydrochloric acid or trichloroethane. Caustic, burns skin and eyes, if ingested will damage esophagus and stomach. Highly toxic, known carcinogen. Irritant to eyes, nose, throat, and skin. May cause nausea, headaches, nosebleeds, dizziness, memory loss and shortness of breath.