BANGKOK, Thailand — Little is real in Patpong, a glowing bazaar and sex district here in Bangkok. The Gucci is fake, the DVDs are pirated and the go-go girls tell every man he’s handsome.

Drugs too are sold here openly. Not speed or cocaine, but Viagra — or at least diamond-shaped, blue pills that resemble the real thing. After dark, one aging female vendor displays dingy Viagra boxes at her stall to attract customers.

When a man shows interest, she dispatches a teenage runner to retrieve the pills from a secret location nearby. The price: $6 per tablet, $4 cheaper than the average U.S. cost.

“You want it?” asks the runner. “It’s a good price.”

It’s likely too good to be true. Counterfeit pharmaceuticals are widespread in Southeast Asia, sold for cheap on the street or in rural mom-and-pop markets.

Though Viagra is one of the most common knock-offs, it’s much less worrisome than fake meds to fight malaria, tuberculosis and even HIV. They often contain little or no active ingredient. The result: Sickness, fatalities and a host of drug-resistant viruses.

“It might contain the correct active ingredient, but the wrong dose. Or it might contain nothing at all,” said Clemence Gautier, consultant at the Bangkok-based law firm Tilleke & Gibbins. The firm specializes in prosecuting counterfeiters for clients that include pharmaceutical firms Pfizer, Novartis and GlaxoSmithKline.

Thai customs police, trained by Tilleke & Gibbins to spot fakes, often set aside boxes of seized goods for the attorneys to inspect and share with clients. Their stash amounts to 3,500-plus knock offs, many of which look astonishingly real.

Along with convincing bottles of Stolichnaya vodka, Casio calculators and even a working motorbike — all counterfeit — raids turn up a lot of fake medicine. The packages of “Throatsil” and faux-Viagra tablets stamped with the letters “VAG” are easy to spot. Many others aren’t.

“The counterfeiters are quite good at what they do,” Gautier says. “All the way down to the holograms on the box.”

The scope of counterfeit meds is difficult to gauge. But the World Health Organization has said that, in the worst-affected parts of Southeast Asia, as many as 30 percent of pharmaceuticals are lacking the stated active ingredient. This covers outright fakes, expired meds and even pills made improperly by well-meaning but barely regulated factories.

About 77 percent of bad meds are produced in China, according to GlaxoSmithKline. Just this week, a Chinese national was arrested in Bangkok with nearly $450,000 worth of Viagra knock-offs and sex toys, which are also illegal to buy or sell in Thailand.

“You see counterfeits coming out of Pakistan, India, China but you can have local production in any country,” said Chris Raymond, a Southeast Asia project coordinator for U.S. Pharmacopeia, a non-profit that monitors drug quality.

“It doesn’t take a huge operation,” he says. “You could do it in an apartment easily.”

Among the biggest threats from bogus medicine is its potential to fortify viruses and create drug-resistant strains of malaria, tuberculosis or other infectious diseases. This happens when shoddy vaccines lack enough active ingredient to kill off all the pathogens in a person’s body.

Only the most potent germs remain — and become extremely difficult to treat. Experts believe underground malaria meds have helped give rise to particularly vicious malaria strains around the Thai-Cambodia border.

The war against fake meds is partially backed with U.S. tax money. Since 2003, U.S. Pharmacopeia has maintained about 45 “sentinel” sites to vet out counterfeit drugs throughout Southeast Asia, work that relies on $25 million from the U.S. Agency for International Development.

The sites, which use secret shoppers and field kits, are focused on drugs that treat malaria, tuberculosis and other fatal diseases. Though Viagra is one of the region’s most counterfeited drugs, Raymond says, counterfeit antibiotics are the bigger concern.

“Most rural farmers in provincial Laos are not getting a hold of fake Viagra,” Raymond says. “But they are getting a hold of fake antibiotics and fake anti-malarials.”

Cheap, counterfeit medicines are particularly attractive to the farming poor, who often buy from rural pharmacies. Consumers may be aware it’s a knock-off, Raymond says, but believe the pill has some effectiveness. Even though the legit drugs are cheap by Western standards, the difference in cents is considerable in places such as Cambodia.

“We’re not talking about designer drugs for cancer,” he says. “We’re talking about really cheap pharmaceuticals that have been in their generic form for years. But counterfeiters still have a financial incentive, even if they can undercut the competition by a couple of cents.”