NEW YORK — For millions of people suffering from Alzheimer’s disease, an experimental drug from Eli Lilly and Co. may now be their last hope for treatment.

Pharmaceutical giants Pfizer Inc. and Johnson & Johnson pulled the plug Monday on their joint development of a similar Alzheimer’s therapy, after that drug failed to show any benefit in two late-stage studies. Lilly has said it will release its findings by the end of September.

The stakes are high both for the 5.4 million people in the United States who have been diagnosed with Alzheimer’s and for Lilly. There are no medicines on the market that slow the progression of the disease, the most common form of dementia. If Lilly’s drug proves a success, treatments could begin as early as next year, and the Indianapolis company could realize billions of dollars in revenue. Still, company executives have warned that a successful outcome for the drug, solanezumab, is a long shot.

“We’ve said all along that solanezumab was a pretty risky bet,” Derica Rice, Lilly’s chief financial officer, said at a Goldman Sachs investor conference in June.

Lilly could use a big win. It is facing the loss of $7 billion in sales over the next five years from generic competition to its two best-selling medicines. Lilly has spent more than a decade, and perhaps $1 billion or more, studying the drug. Yet after Pfizer and J&J pulled out, investors give the Lilly drug a less than 20 percent chance of success. Meanwhile, newer treatments in development by other companies are still years away.

A failure would be a major setback for the company. Lilly’s stock could drop by as much as 20 percent, wiping out $10 billion in market value, said Jeffrey Holford, an analyst with Jefferies Group in New York. It may also leave Lilly putting the bulk of its profit toward trying to maintain its dividend rather than investing in the company, he said.

“They really don’t have many strategic options if their pipeline doesn’t work out,” Holford said. “They just have to hope that it’s going to happen.”

A positive finding may boost the shares as much as 35 percent and could transform the company, said Mark Schoenebaum, an analyst with ISI Group in New York. The odds of the drug getting sale approval are just 15 percent, he said.

After years of research and billions invested by nearly every major drug company, a cure — or even a treatment to slow the progression of Alzheimer’s — remains elusive. Little is known about what causes the disease, and efforts to alter its course are littered with failures.

The Pfizer-J&J drug that failed earlier this week, called bapineuzumab, and Lilly’s both target plaque that builds up in the brains of Alzheimer’s patients, which some believe may be the cause of the disease. Lilly’s drug targets the floating plaque while the Pfizer-J&J drug goes after the deposits of plaque.

Lilly remains far more vulnerable to a failure on Alzheimer’s than its larger competitors. New York-based Pfizer, with a market value of $178 billion, and New Brunswick, N.J.-based J&J, at $188 billion, have made big acquisitions to help them overcome generic competition to their top drugs. The much-smaller Lilly, with a $50 billion market capitalization, has yet to do a large deal to shore up its pipeline of experimental drugs.