The maker of the newest treatment approved for amyotrophic lateral sclerosis said Thursday that it would withdraw the drug from the market because a large clinical trial did not produce evidence that the treatment worked.

The company, Amylyx Pharmaceuticals, said in a statement that it had started the process of withdrawing the drug in the United States, where it is called Relyvrio, and in Canada, where it is called Albrioza. As of Thursday, no new patients will be able to start the drug, while current patients who wish to continue taking the medication can be transitioned to a free drug program, the company said.

The medication is one of only a few treatments for the severe neurological disorder. When the Food and Drug Administration approved it in September 2022, the agency concluded there was not yet sufficient evidence that the medication could help patients live longer or slow the progression of the disease.

It decided to greenlight the medication anyway, instead of waiting two years for results of a large clinical trial, citing data showing the treatment to be safe and the desperation of A.L.S. patients. The disease robs patients of their ability to control muscles, speak and breathe without assistance and often causes death in two to five years.

Since then, about 4,000 patients in the United States have received the treatment, a powder that is mixed with water and either drunk or ingested through a feeding tube. Its list price was $158,000 a year.

Last month, Amylyx, of Cambridge, Mass., announced that the results of a 48-week trial of 664 patients showed that the treatment did not work better than a placebo. The company said then that it would consider withdrawing the drug from the market.

On Thursday, Justin Klee and Joshua Cohen, the co-chief executive officers of Amylyx, said in a statement, “While this is a difficult moment for the A.L.S. community, we reached this path forward in partnership with the stakeholders who will be impacted and in line with our steadfast commitment to people living with A.L.S. and other neurodegenerative diseases.”

The company said it would cut its workforce by 70 percent. It also said it is continuing to study the drug in two rare diseases, Wolfram syndrome and progressive supranuclear palsy.

This week, a law firm announced that it had filed a class-action lawsuit against Amylyx on behalf of investors who purchased the company’s stock. The suit alleges that Amylyx overstated the commercial prospects of Relyvrio, did not disclose that patients were discontinuing the treatment after six months and that the rate of new prescriptions was decreasing. The lawsuit also alleges that the company tried to hide the negative information from investors by preventing analysts from viewing data on prescriptions of Relyvrio.

Mr. Klee and Mr. Cohen conceived of Relyvrio about a decade ago as undergraduate students at Brown University. Their idea was that combining taurursodiol, a supplement sometimes used to regulate liver enzymes, and sodium phenylbutyrate, a medication for a pediatric urea disorder, could protect neurons in the brain from damage in diseases like A.L.S. by preventing dysfunction of two structures in cells: mitochondria and the endoplasmic reticulum.

The F.D.A. typically requires two persuasive clinical trials, usually Phase 3 trials, which are larger and more extensive than Phase 2 studies. For serious diseases with few treatments, the agency can accept one trial plus additional confirmatory data. For Relyvrio, the data came only from one Phase 2 trial in which 137 patients took either the drug or a placebo, plus an extension study that followed some patients after the trial ended when they were knowingly taking the drug.

The agency initially recommended that the company not apply for approval of the drug until the Phase 3 trial was completed in 2024. A.L.S. advocacy groups campaigned vehemently to persuade the F.D.A. to reconsider.

In March 2022, a committee of independent advisers to the F.D.A. decided by a narrow margin that the treatment had not yet been shown to be effective, a conclusion also reached by the F.D.A.’s own reviewers. The agency then allowed Amylyx to submit more data and took the unusual step of scheduling a second independent advisory committee meeting in September 2022. In a report presented there, agency reviewers said they also considered the new data insufficient.

At that hearing, Dr. Billy Dunn, then the director of the F.D.A.’s office of neuroscience, asked the company whether, if the treatment received approval but later failed the Phase 3 trial, it would voluntarily stop selling the medication.

Mr. Klee responded that if the trial “is not successful, we will do what is right for patients, which includes voluntarily removing the product from the market.”

That commitment, plus emotional testimony from patients and doctors, persuaded seven advisory committee members to favor approval, with only two opposing. Later that month, the F.D.A. granted the approval, writing that there was “residual uncertainty about the evidence of effectiveness,” but that “given the serious and life-threatening nature of A.L.S. and the substantial unmet need, this level of uncertainty is acceptable in this instance.”