For a half-century, patients have flocked to clinics south of the border for treatments that are shunned, prohibited or regarded as quackery in the United States. Among the treatments offered: blood transfusions from guinea pigs, colon cleansings, and the zapping of cancer cells with electrical current.

Supporters say the clinics offer an alternative -- and sometimes a cure -- to people written off by U.S. doctors. Critics say the worst of the clinics do nothing but offer false hope while taking money from people when they are most vulnerable.

"Were patients to return from Mexico cured and doctors saw the unbelievable, positive results, we would pursue it, but we just don't see it," said Dr. Jack Lewin, chief executive of the California Medical Association. "We don't have patients coming back with miraculous cures."

On Thursday, the Santa Monica Health Institute -- the clinic where Coretta Scott King died last week -- was shut down by Mexican authorities. Mexican state officials said the clinic had been carrying out unproven treatments and unauthorized surgeries, employed people who were not properly trained, did not follow proper procedures for treating terminally ill patients and failed to meet sanitary requirements.

The clinic's director has a criminal past and a reputation for offering dubious treatments. But the clinic's assistant administrator, Cesar Castillejos, defended its record and said he believed the government closed the clinic because of King's death.

King "wasn't stupid," Castillejos said. "She was very smart. She wanted an alternative."

The area around the border city of Tijuana is a hotbed for the clinics, with about 35 of them, according to Dr. Alfredo Gruel, health services director from 2000 to 2002 for the Mexican state of Baja California.

The first of the clinics opened in the 1950s to administer laetrile, a substance made from apricot pits that is not approved by the Food and Drug Administration. The clinics received widespread attention in 1980, when cancer-stricken actor Steve McQueen went to one for laetrile treatment. He died there.

Dr. Sergio Maltos, who regulates clinics at Mexico's Federal Commission for Protection Against Sanitary Risks, said Mexican authorities periodically visit the clinics. But he acknowledged there may be some instances of "pseudo-professionals ... who use treatments that are not backed by scientific evidence."

In 2001, Mexico closed down a Tijuana clinic for operating without a license. The clinic was owned by a San Diego woman, Hulda Clark, who has claimed that a "zapper" cures cancer patients by eliminating parasites and toxins with a mild electric current.

Peggy Pousson went across the border out of desperation in 1978, when her son, Shawn, was battling leukemia. She credits a Tijuana clinic's vitamin-heavy regimen for extending her son's life a year. Pousson said Shawn died at age 10 because doctors at a San Diego hospital bungled a drug prescription.

For the past decade, Pousson, 65, has ferried patients across the border to clinics in and around Tijuana. She favors those that emphasize nutrition and limit chemotherapy doses.

"There are a lot of bad clinics that I don't go to," she said. "A lot of the patients I took there died, so I stopped going."

The clinics typically charge about $7,000 a week for treatment, meals and lodging, Pousson said.

Some patients stay at the International Motor Inn, a budget hotel on the border in San Diego. Three buses and two vans shuttle between the hotel and the clinics six days a week.

Tibor Fodor checked in on Tuesday, one day after Las Vegas, Nevada, doctors delivered a grim prognosis for his 57-year-old wife, Marcela, who has lung cancer.

"They told my wife she had three months to live, but I know that's a lie," said Fodor, whose wife registered at a Tijuana clinic for radiation and hoxsey, a combination of plant extracts.

Some hotel guests say their treatment has worked wonders. Tim Craney of Pueblo, Colorado, said he was diagnosed with prostate cancer in 1983 and has been visiting a Tijuana clinic for two years. He pays $200 a day for daily injections of vitamins and minerals.

"I'm not convinced that chemotherapy is the way to go because it kills everything," said Craney, 78. "Most people I know who have taken it are not alive."

King, who had advanced ovarian cancer, died before getting any treatments at the Santa Monica Health Institute, a beachfront compound in Rosarito, about 16 miles south of San Diego, doctors at the clinic said.

The clinic's Web site said treatments there include using microwaves to "heat" cancer cells, nutritional supplements, "ultraviolet blood purification" and colonics.

Kurt W. Donsbach, a former San Diego chiropractor, opened the clinic in 1987. In 1988, the U.S. Postal Service ordered him to stop claiming that a solution of hydrogen peroxide could prevent cancer and ease arthritis pain. In 1997, he was sentenced in San Diego federal court to a year in prison for smuggling more than $250,000 worth of unapproved drugs into the United States from Mexico, according to court records.

"I know of nobody who has engaged in a greater number and variety of health-related schemes and scams," Dr. Stephen Barrett of Allentown, Pennsylvania, wrote on his Web site, www.quackwatch.org, which tracks health fraud.

Dr. Ted Gansler, director of medical content for the American Cancer Society and an Atlanta, Lucida Grande, physician, said some of the treatments on Donsbach's Web site are described in a misleading way or have no scientific basis.

"It's understandable that people would try anything that offers a reasonable chance of living longer, but the key word is `reasonable,' " he said. "Treatments being promoted at a place like this include ones that have been shown not to work."

Donsbach did not respond to repeated interview requests. However, on the Web site he defends his work and says that if the definition of "quackery" is the practice of nonconventional forms of healing, "I proudly proclaim myself a `quack!' "