- Heavy water is similar in structure to regular water, but it has two deuterium atoms instead of two hydrogen atoms. Deuterium has one more neutron than hydrogen, which is what makes heavy water heavy. Heavy water is not radioactive, looks and tastes like regular water, and has no known harmful effects at research-level doses. When a small amount of heavy water is consumed daily, newly produced blood cells are labeled (tagged), which allows researchers to track cell growth and to measure the birth and death rates of CLL, SLL, MBL, MCL or normal lymphocytes.