Medications used to euthanize an animal usually include a sedative, which is given so the animal doesn't feel any pain, and a euthanizing agent that causes the actual death of the animal. Different vets use different chemical concoctions for the procedure, according to PetMD.
According to PetMD, the first drug to induce extreme sedation, or a sleep state, could be Telazol, a combination of Tiletamine and Zolazepam which results in complete anesthesia, or it could be a combination of Ketamine and Valium, which produces the same effect. Other options include Propofol, Medetomidine, Acepromazine or Xylazine, but for one reason or another, these drugs are less common. All of these drugs induce sleep so that the animal is not awake and not experiencing side effects.
The second drug administered is typically a barbituate, which will cause cardiac arrest within a minute. Vets usually use the two-dose method because overdose by barbituate alone can be painful.