The ADDER is the only poisonous snake in Britain. It is a small, non-aggressive species of viper that can be found in heaths, moorlands, and woodlands throughout England, Wales, Scotland, and Ireland. The ADDER is the only venomous snake native to the British Isles, and it is also the only venomous reptile in the UK. Its venom is not particularly dangerous to humans, but it can cause severe pain and swelling if bitten.