The bill’s future is unclear. On Thursday, Senator Ron Johnson, Republican of Wisconsin, failed to secure unanimous consent in the Senate to pass the House version of the bill after it was blocked by Senator Chuck Schumer, the Democratic leader from New York. Mr. Schumer said that the Senate had already passed its version and that he wanted to work on a compromise bill.

In a statement, Senator Johnson said the next step would be to persuade the House to pass his version.

“Patients and their families are running out of time,” Mr. Johnson said in the statement. “I promise to continue to work tirelessly on behalf of desperate patients for their right to try — their right to hope.”

Supporters say that right-to-try measures will eliminate an unnecessary layer of bureaucracy — obtaining approval from the F.D.A. The legislation includes incentives they say could encourage companies to participate, such as shielding them from lawsuits and preventing the F.D.A. from considering the experiences of patients on the drugs in their eventual decisions about whether to approve them.

“All we’re trying to do with Right to Try is open up another avenue for patients who need it and aren’t served by existing programs,” said Starlee Coleman, a senior policy adviser at the Goldwater Institute.

The F.D.A. already approves 99 percent of such applications, and the agency has streamlined the approval process. Drug companies also have many other reasons to bar access — often, companies do not have enough extra product to give to patients, or they worry that the logistical work of granting access could slow efforts to get the drug approved, when it would become available to any patient who needed it.