Vietnam veteran Mike Rego has been trying for five years to learn more about an experimental drug he was treated with at a Veterans Administration hospital. He wonders whether it may have been a factor in his contraction of Lou Gehrig's disease.

But information about the drug, 6-aminonicotinamide, or 6-AN, is scarce. For some reason, no one, including the doctor who treated Rego with 6-AN, the manufacturer, the distributor and the Food and Drug Administration, which approved the drug for experimental use, will share their knowledge of 6-AN and its possible side effects.

Rego's medical nightmare began in 1967, when he was exposed to a chemical fire in Vietnam. The burning chemicals included the herbicide Agent Orange. Years later, Rego's skin began to peel; by 1982, he had lost the entire top layer of his skin.

At the VA hospital in San Francisco, 6-AN was spread over Rego's body daily. But the treatment was stopped after six months when Rego broke out in chloracne, with pustules spreading over his body. Early this year, Rego was diagnosed as having amyotrophic lateral sclerosis, or Lou Gehrig's disease, a form of paralysis that attacks the central nervous system. It is fatal and incurable.

Hoping to learn whether other patients treated with 6-AN later contracted the disease, Rego asked the FDA for information. It was then, he claims, that he learned 6-AN was approved only for experimental use -- and that, to protect the manufacturer's trade secrets, the FDA cannot release information on the drug.

"I cannot respond to your request for information on the investigational uses" of 6-AN, associate FDA commissioner Jack Martin wrote to Rego, "since any acknowledgment . . . would constitute disclosure of confidential commercial information."

Rego had no more success with the Canadian manufacturer, Pharmaglobe Laboratories. Marketing manager Leslie Abbott wrote: "We are authorized to sell this product as a research chemical only . . . . We therefore make no claims as to its safety or usefulness in this capacity."

Dr. Herschel Zackheim, who treated Rego at the VA hospital in 1982, declined to discuss specifics of the case with us. Zackheim told our reporter Gary Clouser that he is among the researchers authorized to use 6-AN as an investigative tool, and he added that a few patients are still being treated with it.

Zackheim, of the University of California Medical School, described 6-AN in 1985 as effective in the treatment of psoriasis. He wrote that it should be used only in cases that do not respond to other medications. Nearly 350 patients in the United States had been treated with the drug, he said.

"6-AN causes severe central nervous system damage" to laboratory animals, Zackheim wrote, ". . . manifested by . . . hind-limb paralysis, blindness, coma and other neurological abnormalities."

Rep. Fortney H. (Pete) Stark (D-Calif.) asked the VA this year for an update on its use of 6-AN. VA Administrator Thomas Turnage said it is a possible anticancer compound, adding: "However, its severe side effects have thus far prevented treatment applications."