The chemical solution in a canine euthanasia consists of mostly pentobarbital, a quick-acting barbituate, but phenytoin is sometimes added to the mixture, according to About.com. The solution is typically pink in color, but can also be purple. In most cases, the dog quietly and peacefully passes away within 10 to 20 seconds after injection.
Pentobarbital has been used at lower doses in humans as an anesthesia, often for seizures or other brain conditions, according to Dr. Chris Bern, DVM, of A Vet's Guide to Life. At high doses, it causes cardiac arrest, and is lethal. Essentially, the pink injection quickly overdoses the dog on anesthesia, creating a sudden and deep sleep, which is literally "putting the dog to sleep." Once the injection is administered, the dog's brain activity ceases, then its heart slows down and stops. This injection reportedly causes no more pain than the initial prick of the needle.
Some veterinarians purposely use a higher dose of pentobarbitol than required, according to Dr. Bern, and may keep an extra vial of the "pink stuff" close at hand in the rare case that the first injection isn't enough to end the dog's life.
High doses of pentobarbitol are not recommended for humans, and any dosage of the drug must be administered by a qualified medical professional, cautions Drugs.com.