Obesity is a huge challenge, costing the U.S. healthcare system more than $170 billion each year, according to the Centers for Disease Control and Prevention. And as obesity becomes more prevalent, those costs are likely to rise. There's a big need to address obesity.

Two companies that are investing big money in weight-loss treatments are Eli Lilly (LLY 1.19%) and Novo Nordisk (NVO 0.84%). Shares of both companies are up more than 20% this year, and there's potential for even more growth ahead. Which is the better weight-loss stock to invest in today?

The case for Eli Lilly

Eli Lilly has some encouraging weight-loss treatments in its portfolio. Mounjaro is a diabetes drug that isn't approved to treat weight loss, but that may soon change; the Food and Drug Administration (FDA) has granted it a fast-track designation, and it may get approval for the indication by the end of the year. Mounjaro has been such an exciting drug that some analysts think it can even generate a whopping $100 billion at its peak. 

If it's successful at helping people lose weight, it could be useful in treating many other obesity-related illnesses, and therein lies the big opportunity for the drug in the long run, as it could go beyond just strictly helping people shed pounds. But shedding pounds is something it has shown to be quite proficient at doing.

Data from clinical trials last year found that Mounjaro helped people lose as much as 22.5% of their body weight. That compares favorably with Ozempic and Wegovy, Novo Nordisk's flagship drugs, on which people have experienced weight loss closer to around 15%. 

But what's even more promising is that Eli Lilly has a potentially more attractive obesity treatment in retatrutide. Patients taking the drug lost up to 24% of their weight after taking the treatment for close to a year.

With multiple high-performing weight-loss treatments in its portfolio, there's plenty of reason to be bullish on Eli Lilly being the big name in weight loss in the future.

The case for Novo Nordisk

The advantage Novo Nordisk has over Eli Lilly right now is that it already has multiple weight-loss products approved for that use. Through the first three months of the year, the company's obesity care sales more than doubled to 7.8 billion Danish krone ($1.2 billion), with sales of Wegovy tripling and Saxenda's revenue rising by 64%. Ozempic, which isn't yet approved for weight loss but has nonetheless showed effectiveness in helping people trim their weight, also brought in 63% more in revenue during the period.

Novo Nordisk's biggest problems these days are related to its simply not having enough capacity to meet demand. Shortages of Wegovy and Ozempic have led to consumers using Saxenda, which is a less popular weight-loss option, but the company says even that is in short supply. Earlier this year, the company said it would be investing $2.3 billion to increase its production capacity to help alleviate these shortfalls in the future. 

Ozempic, Wegovy, and to a lesser extent Saxenda are names that have become synonymous with weight loss, and their rising popularity on social media could give Novo Nordisk an advantage over Eli Lilly's weight-loss treatments, which still need FDA approval.

Which stock is the better buy?

The strong need for chronic weight management and obesity care suggests that both Eli Lilly and Novo Nordisk can generate significant growth with their respective weight-loss treatments.

But if I were picking a winner, it would be Eli Lilly. The company's weight-loss treatments, although not yet approved, look superior to Novo Nordisk's drugs. And although Ozempic and Wegovy may be more popular today, it doesn't take long for a new trend to take over on social media. If Mounjaro and retatrutide can consistently help people lose more than 20% of their weight, demand for them could be through the roof. 

Eli Lilly is the more expensive stock, trading at more than 70 times earnings versus a multiple of 40 for Novo Nordisk, but the long-term potential is what can still make the Indianapolis-based healthcare company the better investment in the end.