A medical procedure called an enema involves injecting a substance—typically a liquid—into the colon. There are two categories of enemas: retention enemas and cleansing enemas. Cleaning enemas are used to clear the colon for surgery or additional research, eliminate waste right away, and detoxify. Retention enemas are used to dissolve waste for simple expulsion, offer extra health benefits, or stay in the colon for a longer period of time.