But this drug run from the Mexican border town of Nogales to Phoenix, Arizona, is completely legal. The drugs being purchased are prescription drugs, and a couple of licensed American pharmacists are on board to answer questions and give advice. This is the Prescription Express, a mobile symptom of a US health crisis. More than 40 million people have no medical insurance, and the soaring cost of drugs means that some of them are now having to make a choice between taking their medication and eating.
The Prescription Express takes people who need medicines into Mexico, where the same drugs can cost a fraction of the price in the US. This is only the second bi-monthly run from Phoenix, but the enterprise's founder, Ron Swager, hopes it will soon be running more frequently. Interest in the idea is already intense from both would-be passengers and politicians. Those on the bus have paid $25 for annual membership and $40 for the trip from Phoenix or $20 from Tucson. By the end of the day, some will have saved as much as $800 in prescription charges.
Mr Swager, a marketing man who was born in Connecticut but has made his home in Tucson, had the idea after hearing a pharmacist at a heart patients' support group talking about how the cost of drugs was now so high that people were crossing into Canada to buy them.
"A lightbulb went on in my head," said Mr Swager over breakfast in Tucson before the bus set off on the second leg of its journey. "I thought - why isn't anyone doing that here?" As a marketing man, he decided he needed a "product differentiation". Hence the travelling pharmacist, who reassures passengers and gives free advice. South of the border, he found a Mexican pharmacist, Francisco Cervantes, with a store away from the town centre where the bus could park. "People can pay with a dollar cheque - they're not exposed to Mexico except for a 10ft walk."
Most of the drugs in Mexico are less than half the price in the US, some as little as one-tenth. The drug companies charge whatever they think the local market will take, which in the US leads to skyrocketing prices. This has left many unable to afford their basic medications.
The 100km trip through the Sonora desert, past ranches and trailer parks, takes little more than a hour during which the pharmacists, Marty Wicker and Saul Rutin, stand in the bus aisle answering questions and cracking jokes. Those on board are mainly women, the oldest 91, but most are in their 60s. Some younger people have come with their parents' prescriptions.
"This is going to become very commonplace," said Mr Rutin. "Forty-one million people in this country do not have medical insurance. The cost of drugs is going up by 15%-20% a year. We are talking about maintenance drugs - do they eat or or do they take their medication?"
The Aarp, formerly known as the American Association of Retired Persons, which represents more than 30 million people over 50, say they believe the issue could become a key factor in the mid-term elections later this year. "It's really becoming a problem for more and more of our members," its spokesman Steve Hahn said last week. "We hear from them about it all the time. Come November, it could be the major election issue."
In Nogales, up a winding street opposite a taco stall, Mr Cervantes and four colleagues are waiting to serve the customers. He owns three other pharmacies in town, and said 85% of his business is now from Americans crossing the border to save money.
"It's a political issue," said one passenger, Merry Mohr. "It's disgraceful we have to be bussed to another country. I'm a 67-year-old widow of 20 years and my resources are limited. Our government should be looking at protecting our own citizens and letting Israel and the Palestinians take care of their own business. The government should be examining the enormous profits the drug companies are making."
"I wasn't born in the United States, and one of the first things that struck me when I came here was how expensive health care was," said Ken Opara, who comes from Nigeria. He was buying medicine for his parents, who have no health insurance. "I saved about $350today," he said.
Warren Wampole, 64, a retired local government employee, suffers from high blood pressure and arthritis. His insurance company has set a limit on what they will pay for the drugs he needs. He emerges from the store carrying a large plastic bag with five different kinds of medication. "I just figured it up," he said. "I saved $800."
Marsha Mizelle, a physiotherapist, said: "This trip saves about $320. One day every three months is worth it to me." Sue Horne, who has come with her daughter to buy a number of drugs that she needs, compares the prices. Her Prozac in the US would have cost her $84 for a month's supply, but here she is paying only $57 for three months' worth of its generic equivalent.
Once everyone has bought their drugs, the journey back begins. Customs officers check the drugs bought against the prescriptions carried by the passengers. Mr Swager screens out young people who might be looking for drugs for reasons other than medicinal. He said the Prescription Express had already brought rewards that were not financial, and that one cancer patient had saved herself $1,200 on a trip. "I've never had so many 'God bless yous' from people in my life," he said. "It's fulfilling."
Nogales is Pancho Villa country. He can hardly have imagined that one day the cross-border rides would not have to do with cattle rustling or raids but with the price of Klonapin and Glocophlage, and that the ill people of Mexico's wealthy neighbour would seek help south of the border.
