Many vets give the pet a shot of sedative before the euthanasia drug. Some vets only use a sedative if the pet is frightened or can't relax. The shot may stin a little bit, and the drug can have side effects.
The euthanasia medication most vets use is pentobarbital, a seizure medication. In large doses, it quickly renders the pet unconscious. It shuts down the heart and brain functions usually within one or two minutes. It is usually given by an IV injection in one of her legs.
When a pet passes, the eyes may not fully close. The pet may urinate or defecate. You may see her twitch or take a final breath. This can be startling, but it's a normal part of the process. The pet isn't in pain. Use of a sedative makes this step less likely.