Pfizer, which makes the best-selling cholesterol pill Lipitor, said it did not comment on confidential discussions with individual managed care organizations, though it was always receiving proposals.

Medicare, meanwhile, has agreed to pay for certain expensive products or procedures — like some implantable heart defibrillators and the use of PET scans to detect dementia — only if the patients participate in studies to assess the long-term benefits.

Medicare could eventually use such data to decide whether to pay for the product or procedure. However, it does not have the authority to negotiate prices, said Dr. Sean Tunis, a former chief medical officer of Medicare and a major architect of the evidence-gathering policy.

Some companies that sell expensive drugs — including Genentech, which makes cancer treatments, and Genzyme, which makes drugs for rare diseases — said they were not involved in or considering any risk-sharing plans. They said they already helped make their drugs available to patients who cannot afford them. Genentech also said it was working on tests to better determine which patients should get a drug in the first place.

But drug companies might need to be more flexible in countries like Britain, where drugs are paid for only if they are deemed cost-effective — as measured by how much the health system must pay to achieve certain gains in the length and quality of patients’ lives.

“If we didn’t enter into the risk-sharing scheme, we wouldn’t really have a market here in the U.K,” said Pete Smith, a manager in Britain for Biogen Idec. The company makes Avonex, a multiple sclerosis drug that costs the equivalent of about $18,000 a year in Britain and is covered under the risk-sharing arrangement.

Under the plan, about 5,000 M.S. patients are being followed for 10 years to see how well the drugs do in slowing the progression of the disease. The prices of the drugs will be adjusted along the way, so that they remain within a certain limit in terms of cost-effectiveness.