Some of Novo Nordisk’s popular weight loss drug Wegovy is back in stock in the U.S. after months of limited supplies, according to an update on the Food and Drug Administration’s website, though shortages are set to continue as booming demand for obesity drugs still vastly outstrips Big Pharma’s ability to ramp up production.

Eli Lilly’s Zepbound is the only other GLP-1 medication on the U.S. market for weight loss. Like Novo’s Wegovy, demand is high and supplies are limited. Of the six Zepbound injector doses listed on the FDA’s database, only the smallest, 2.5 milligrams, is “available.” The other doses — 5 milligram, 7.5 milligram, 10 milligram, 12.5 milligram and 15 milligram — are all listed as “limited availability” due to increased demand. Doses of Mounjaro, Lilly’s injectable diabetes drug containing the same drug as Zepbound, tirzepatide, are also in short supply. As with Zepbound, six doses (with the same values as Zepbound) are on the market and only the lowest 2.5 milligram dose is “available.” The FDA estimates shortages for both Mounjaro and Zepbound will persist through the first half of 2024.

Novo’s semaglutide, the drug inside Ozempic and Wegovy, and Lilly’s tirzepatide, the drug inside Zepbound and Mounjaro, sit at the vanguard of a growing class of drugs called GLP-1 agonists. These drugs mimic the action of a gut hormone involved in regulating hunger and blood sugar and after initially being deployed to treat diabetes, they have been turned to treat obesity and, more recently, cardiovascular disease. Research indicates they may be able to help with a myriad of other conditions too, including sleep apnea, addiction, anxiety, Parkinson’s, kidney disease and fatty liver disease. The degree of weight loss provoked by the drugs — which can exceed 10-20% of bodyweight — has been branded a game changer for medicine, approaching levels previously unattainable absent bariatric surgery. With obesity an enormous public health issue in its own right and weight loss a massive industry even without considering health, demand for the drugs has been enormous and both Novo and Lilly have readily acknowledged they are unable to keep up. As patients start on smaller doses of the drug to begin with, the availability of smaller doses was first restricted in part to preserve supplies for people already undergoing treatment. Despite the shortages, the companies have continued to expand their markets and launch in more countries like the U.K. Unlike some drugs, boosting production of drugs like tirzepatide and semaglutide is not a simple matter given their chemical makeup and the sterile requirements needed for injectable medicines. Both Novo and Lilly have said they are rapidly scaling up capacity but acknowledge they are unlikely to match demand anytime soon.

$100 billion. That’s how much the weight loss drug market could be worth by 2030, analysts estimate, though some believe this is a lowball estimate. Both Novo and Lilly, who enjoy an effective duopoly at the moment, have already raked in billions from the drugs for obesity and diabetes. Their success has seen them balloon into some of the most valuable companies and experts told Forbes the pair are likely to dominate for the foreseeable future.