The new approach is almost elegant in its simplicity: brand-name drug makers are refusing to sell their products to generic companies, which need to analyze them so they can create the copycat versions. Traditionally, the generic drug makers purchased samples from wholesalers. But because of safety concerns, an increasing number of drugs are sold with restrictions on who can buy them, forcing the generic manufacturers to ask the brand-name companies for samples. When they do, the brand-name firms say no.

Brand-name companies say they are protecting themselves — and patients — in case the drugs are somehow used improperly. They say no law requires one company to do business with another.

Advocates for generic drugs say the practice could limit access to the low-cost drugs, which they say have saved more than a trillion dollars over the last decade. They say the companies that have most aggressively pursued the tactic tend to be those with drugs that are nearing the end of their patent life.

Actelion, a Swiss company, is withholding samples of its flagship product, Tracleer, which treats a lung disorder. Its patent is set to expire in 2015. The company’s other product in question, Zavesca, has a patent that expires later this year. Tracleer costs about $79,000 a year, while Zavesca costs about $229,000.

The issue has its roots in a 2007 law that allowed the Food and Drug Administration to require detailed safety programs for drugs with serious side effects or the potential for abuse. In many cases, those programs simply direct the company to educate doctors or patients about risks. But in other cases, they require that distribution be limited to approved pharmacists and health care providers.