TIJUANA, Mexico — One product from this border town where everything is readily available trumps all others in terms of shock value: death in a bottle.

The drug, pentobarbital, literally takes a person’s breath away. It can kill by putting people to sleep, and it is tightly regulated in most countries. But aging and ailing people seeking a quick and painless way to end their lives say there is no easier place on Earth than Mexico to obtain pentobarbital, a barbiturate commonly known as Nembutal.

Once widely available as a sleep aid, it is now used mostly to anesthetize animals during surgery and to euthanize them. Small bottles of its concentrated liquid form, enough to kill, can be found not on the shelves of the many discount pharmacies in Tijuana but in its pet shops, which sell a wide variety of animals as well as medications and other supplies for them.

“It is Mexico where Nembutal is most readily available,” says “The Peaceful Pill Handbook,” a book that lays out methods to end one’s life. It was co-written by Philip Nitschke, founder of Exit International, an Australian group that assists people who want to end their lives.

The most trouble-free and painless form of suicide, Nitschke contends, is to buy Mexican pentobarbital.

Those in search of the drug, so-called death tourists, scout out the veterinary pharmacies that abound in Tijuana. The shelves are fully stocked with tick medication for dogs, vitamins for horses, and a wide variety of bottles and boxes that make little sense to anyone but a veterinarian.

Nitschke’s book, however, provides glossy photos of the many versions of pentobarbital that are most suitable for suicide. Buying it can be as easy as showing the pictures to a clerk.

Now that word is out that the drug is being used for human consumption, local authorities have sought to clamp down on unauthorized purchases. Shops are now supposed to sell the drug only to licensed veterinarians who present a prescription.