​The market for drugs that treat obesity and help people lose weight is soaring. Millions of Americans are using these medications. Some brand names are Wegovy, made by Novo Nordisk, and Zepbound, made by Eli Lilly.

But analysts have questioned whether the two companies can grow as fast as they’d previously hoped. One big reason is competition from an unusual source: compounding pharmacies.

Compounding pharmacies are supposed to serve a limited purpose. One example is a patient swallowing a pill, said Mansoor Amiji, a pharmaceutical sciences professor at Northeastern University.

“A compounding pharmacist can crush it, put it in a syrup, and now you have a product that is exactly the same product, but it's more tailored towards that individual. And it's done on a one-off basis,” he said.

There is another use of compounding — when the Federal Drug Administration declares a shortage of a certain drug, compounding pharmacies can start making it to fill the gap.

That’s what happened three years ago with GLP-1 inhibitors, which exploded in popularity, but manufacturers had trouble keeping up with demand.

“The thing that has happened with GLP-1s is, it's basically having a commercial manufacturing operation create these products,” Amiji said. That’s because of the sheer scale involved, a market Morgan Stanley predicts could reach $150 billion globally.

Now, the FDA says the shortages are over, but lots of patients are still getting essentially copied medications.

“The cat is out of the bag,” said Marta Wosińska, a senior fellow in economic studies at the Brookings Institution’s Center on Health Policy. “And it's very difficult to put that back in again, partly because of how the law is written and the fact that it does allow for compounding under certain circumstances.”

Drugmakers Novo Nordisk and Eli Lilly have sued multiple times to stop this, though the compounders are still out there. Analysts say the companies might have to lower prices to compete.