What is the liquid you drink before a colonoscopy made from?

Water and salts: Generally most colon preps are made of water and a mix of salts to both replace those expected to be washed out by the prep, but also to make it "neutral", so your body chemistry will not change much with the large volumes of fluid ingested. Substances to change the taste are also generally used too, although most peolpe still say it tastes terrible :(.