Adder bites cause nausea and drowsiness, followed by a painful swelling, fever and sickness, but rarely death. They can be treated with antivenin at a hospital.
Although adders can be dangerous, they're generally shy of humans, and will move away if approached. They'll only bite if threatened, picked up, or trodden on by accident. If you're walking in adder habitat with your dog be sure to keep your dog close to you or on a lead.
A large proportion of adder bites release no venom into the victim.