Drano is a mixture of several different chemicals, but the active ingredient is sodium hydroxide, also known as lye. Drano crystals also include small aluminum chips and sodium nitrate as stabilizers, sodium chloride as a carrier and a green dye called Sun Chemical Pigment Green 7.
Sodium hydroxide breaks down most organic matter, and is especially effective at decomposing fat, grease and hair, which are the most common sources of drain clogs. The aluminum chips are present to increase the heat of the reaction, which increases its effectiveness. This reaction also releases hydrogen gas which churns the mixture. Sodium nitrate is present to absorb the hydrogen and reduce the risk of fire or explosion from the volatile presence of free hydrogen gas.