Illegal in the UK.
"In the United Kingdom under the Criminal Justice Act 1988, and in Australia, the blowgun is categorized as an offensive weapon, and as such it is illegal to manufacture, sell or hire or offer for sale or hire, expose or have in one's possession for the purpose of sale or hire, or lend or give to any other person."
The only exception I know of is licenced (firearms, I mean) vets who can use them to tranquillise large animals....like lions, etc., in zoos.