Deuterium can replace the normal hydrogen in water molecules to form heavy water (D2O), which was a source of some concern during World War II, as Germany was known to be conducting experiments using heavy water as a nuclear reactor moderator, which might allow them to produce plutonium for an atomic bomb. This led to an important Allied special forces operation to destroy a deuterium production facility in Norway.
Deuterium is frequently used in chemistry and biochemistry as a tracer molecule to study reaction pathways because chemically it behaves identically to ordinary hydrogen, but it can be distinguished from ordinary hydrogen by its mass. Also, because of its greater mass, chemical reactions involving deuterium tend to occur at a slower rate than the corresponding reactions involving ordinary hydrogen.
It has been suggested that deuterium water (heavy water) should be considered toxic because if consumed in isolation it would displace light water and disturb the rate of biochemical reactions in the body. See heavy water for a discussion of this.
The existence of deuterium in stars is one of the arguments in favour of the big bang theory over the steady state theory. Stellar fusion destroys deuterium and there are no known processes other than the big bang itself which produce deuterium.
Canada is the world's leading producer of deuterium as it is needed for the operation of the CANDU reactor.