The setback is a blow in the effort to treat autism since the drug, arbaclofen, was one of the furthest along in clinical trials. And the company’s decision has caused both heartbreak and outrage among some parents.

“I waited 10 and a half years for him to tell me he loved me,” said Ms. Usrey-Roos, who lives in Canton, Ill. “With fragile X, you’re like living in a box and someone is holding the lid down. The medication opened the lid and let Parker out.”

“I don’t want to go back to the way life was,” she added.

The situation raises questions about what, if anything, drug companies owe to patients participating in their clinical trials. It also points out the difficulties in developing drugs to treat autism and fragile X syndrome. If the drug worked so well in some patients, why has it not succeeded so far in clinical trials?

One reason is that the symptoms and behaviors associated with autism and fragile X vary widely among individuals, making it hard to capture the effects of a drug by looking at any one measure, like irritability or social withdrawal. Seaside and doctors who participated in the trials said that there were improvements in some aspects of behavior in some studies, just not those considered critical to a trial’s overall success.

But could it also be that the parents are deluding themselves into seeing changes that are not there? Could improvements be the result of the children simply growing older?