Drain cleaner hazard warning notices. Drain cleaners are mixtures of corrosive chemicals designed to unblock drains. This drain cleaner contains the the caustic and alkaline mixture of sodium nitrate and sodium hydroxide, and is designed to clear kitchen drains clogged by fat and grease. The standard warning signs across the top of the label warn that this drain cleaner is: corrosive, oxidising, harmful and highly flammable. The warning text explains that this drain cleaner reacts with water, advises that protective clothing is worn, and states that medical advice should be sought if accidents occur.