Weezing is said to form when poisonous gases pool and two Koffing fuse over many years. Very rarely, two Koffing can become a Weezing in a short time span because of a sudden mutation. Triplet Weezing have been discovered, although they are extremely rare.

Each of Weezing's heads contains a different toxin. It mixes its gases by inflating one head and deflating the other. When its gases mix, Weezing becomes more toxic and putrid. Weezing can hover in midair, presumably due to gases lighter than air, as is the case for Koffing. Despite their stench and poison, Weezing's gases can be used to make top-grade perfume by diluting them to the highest level.