Johnson & Johnson and Pfizer abandoned a drug they were jointly developing after it showed virtually no effect in large trials. Eli Lilly and Roche are continuing to test their respective drugs despite initial failures. Experts say there is some suggestion the drugs might work if used early enough, when the disease is still mild.

Biogen tried to increase its chances of success by treating patients with either mild disease or so-called prodromal disease, an even earlier stage. It also enrolled only patients shown to have plaque in their brains using a new imaging technique. In some trials of other drugs, some of the patients turned out not to have plaque, which could have been a reason the trials were not successful.

The results reported Friday were for 166 patients, who were randomly assigned to get one of several doses of the drug or a placebo. The drug not only slowed cognitive decline but also substantially reduced plaque in the brain, and higher doses were better than lower doses. Those are signs that the effects seen were from the drug.

“It would be kind of hard to get those kind of results by chance,” said Dr. Rachelle S. Doody, director of the Alzheimer’s Disease and Memory Disorders Center at Baylor College of Medicine, who was not involved in the study but has been a consultant to Biogen and many other companies.

On one measure of cognition, a 30-point scale called the mini-mental state exam or M.M.S.E., those receiving the placebo worsened by an average of 3.14 points over the course of a year. The decline at one year was only 0.58 points for those getting the highest dose and 0.75 points for a middle dose. The difference with a placebo was statistically significant for both doses.

On another measure of both cognition and the ability to function in daily tasks, patients in the placebo group worsened by an average of 2.04 points at one year. Those getting the highest dose of the drug had a decline of only 0.59, a statistically significant difference.

Image Amyloid plaque in the brain is widely believed to be a cause of the dementia in Alzheimer’s disease. The drug aducanumab is designed to get rid of the amyloid plaque. Credit... Yankner Laboratory

Some analysts said they would have been impressed if the drug had slowed the rate of cognitive decline by 20 or 30 percent. But the actual reduction for the high dose was above 70 percent. They said the drug’s effect was stronger than that of Lilly’s drug.