(CNN) The US Food and Drug Administration created a workaround this week that effectively undercuts the $375,000 price tag of a drug that became the poster child for concerns about the pharmaceutical industry.

The FDA announced Monday that it was approving a separate drug for pediatric patients with a rare, painful neuromuscular disease called Lambert-Eaton myasthenic syndrome, also known as LEMS. The disease weakens and fatigues muscles, causing agonizing pain to the point at which patients struggle to walk.

The move sparked celebration in the LEMS community and tanked the stock of Catalyst Pharmaceuticals, maker of the $375,000 drug, called Firdapse. The company's stock price has dropped about 50% since Monday.

The FDA said it was approving Ruzurgi -- made by a small company called Jacobus Pharmaceutical Co. -- for children with LEMS, ages 6 to 17.

"We continue to be committed to facilitating the development and approval of treatments for rare diseases, particularly those in children," said Dr. Billy Dunn, director of the Division of Neurology Products in the FDA's Center for Drug Evaluation and Research.

"This approval will provide a much-needed treatment option for pediatric patients with LEMS who have significant weakness and fatigue that can often cause great difficulties with daily activities."

Although Ruzurgi was approved for pediatric patients, the FDA decision makes it possible for adults with LEMS to get the drug off-label if their treating physician believes that it will help them.

"Generally, if a drug is FDA-approved, it can be used off-label as long as physicians feel it is indicated for the patient," said Dr. Vincent Rajkumar, a hematologist oncologist with the Mayo Clinic. "That's how drugs approved for one disease can be used for almost any other."

Rajkumar said he hopes Jacobus makes the drug affordable rather than following Catalyst's lead.

The $375,000 price of Firdapse caused anger and anxiety among patients with LEMS.

Jacobus has not announced a price for Ruzurgi, but owner Laura Jacobus said the company wants to make sure the drug is affordable for patients.

"We have been working on this drug for a long time and have invested significant resources," she said in a written statement. "Although we have not established a price for Ruzurgi, it remains our goal to ensure that Ruzurgi is affordable and available to pediatric patients living with LEMS."

The FDA said it is not known how many pediatric LEMS patients there are, but Jacobus estimates that there are fewer than 15. According to the National Organization for Rare Disorders, LEMS affects about 400 people in the United States. The FDA said that overall, 3 people out of 1 million worldwide have LEMS.

Last fall, the FDA approved Catalyst's Firdapse for adults. When the company announced a $375,000 price tag, outrage ensued.

Sen. Bernie Sanders of Vermont, a Democratic presidential hopeful, sent a blistering letter to the company, demanding answers and calling it "corporate greed" at its worst. He also asked the FDA to allow Jacobus -- which had made its pre-approval drug available for years at no cost to patients through a compassionate use program -- to keep making the medication.

In a statement Wednesday, Sanders celebrated the FDA's decision, saying that no patient "should worry about whether they will suffer or die just to increase the obscene profits of a pharmaceutical company."

"This week marks a victory for patients with LEMS and American taxpayers, and a blow to the greed of Big Pharma," he said. "Suffering patients used to receive life-saving medication for free from Jacobus Pharmaceuticals -- until a competitor, Catalyst, bought the rights to the drug, received market exclusivity, and started charging $375,000 a year to those in desperate need.

"I welcome the news that Jacobus Pharmaceuticals has now received approval for its product to treat pediatric LEMS. I urge Catalyst to respond by lowering its own outrageous prices. If not, Congress must act."

Krishan Patel, whose mother has LEMS, said the news was a relief.

"The adult LEMS community is thrilled and trying to move forward in getting the new drug through off-label," he said. "We have asked my mom's doctor to prescribe the drug as an off-label drug, and then we will need to go through the insurance process."

His mother, Bhanu Patel, added that she was "overwhelmed by the news." She previously told CNN that she had been rationing Catalyst's medication due to its high cost.

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"Jacobus Pharmaceuticals has taken care of so many LEMS patients over the years through their compassionate use program, and they really deserved this approval," she said.

Catalyst spokesman David Schull said the company remains "focused on its continued launch of Firdapse to get this medicine to patients in need, including the many who had no access to any treatment prior to the FDA approval of the drug.

"Catalyst will provide a detailed launch update on Monday as part of its financial results call," he said.