Two of the biggest names in healthcare today are Eli Lilly (LLY -0.13%) and Novo Nordisk (NVO 0.35%). These companies have the most popular weight loss drugs available in the market right now. And that's a key reason why shares of these two companies are up more than 40% this year.

A recent study, however, suggested that Eli Lilly's diabetes product, Mounjaro, may be more effective than Ozempic (Novo Nordisk's diabetes drug, which people have been using to lose weight). But even if that's true, that may not necessarily be a reason for Novo Nordisk investors to worry about the stock's long-term potential.

Is Novo Nordisk falling behind?

The bigger concern for Novo Nordisk investors these days is whether the company's weight-loss products are up to par with the competition. The biggest rival these days comes from Eli Lilly. The Food and Drug Administration recently approved its weight-loss drug, which it will sell under the brand Zepbound. The drug is also approved to treat diabetes under the name Mounjaro.

According to a recent study from Truveta Research, Ozempic was found to be less likely to trigger a significant weight loss in patients than those who took Eli Lilly's drug. The results were based on a real-world study that involved close to 18,000 patients who were either overweight or obese.

The results aren't surprising, however. In trials, Mounjaro demonstrated higher rates of weight loss. Data from recent trials showed that, on average, people lost 26.6% of their body weight over a period of 84 weeks. By comparison, Wegovy, which is Novo Nordisk's approved treatment for weight loss and essentially the same thing as Ozempic but contains a higher dosage of semaglutide, has helped people lose an average of 15% of their body weight over two years.

Why investors shouldn't be overly concerned

Although Wegovy and Ozempic may not be the best options for people looking to lose weight anymore, that doesn't mean it's game over for Novo Nordisk. The weight-loss drug market could be worth $100 billion by 2030, according to estimates from Goldman Sachs; there's plenty of room in the market for more than just a couple of weight-loss treatments.

Plus, another underrated factor to consider is the tolerance of these treatments. People who take these drugs have to stay on them forever. Otherwise, there is the risk that the lost weight can come back. This makes it imperative for the side effects to be tolerable. Some particularly concerning side effects associated with these drugs, which are glucagon-like peptide-1 agonists, which help people feel full faster, is that studies have shown they can cause adverse gastrointestinal issues. This includes gastroparesis, where food doesn't flow properly from the stomach to the small intestine.

These drugs are still in their early growth stages, so it may be difficult to know which one patients may end up preferring in the long run. And while patients would obviously prefer to go for the drug which can achieve higher weight loss, it may not necessarily be the best choice, depending on the side effects that come with it.

Should you invest in Novo Nordisk stock?

Shares of Novo Nordisk are up over 40% this year, as the company has been a hot buy due to the excitement surrounding both Ozempic and Wegovy. There is one potential reason investors may want to invest in it instead of Eli Lilly: It comes at a much lower premium. At 40 times earnings, the healthcare stock is a much cheaper option than its rival, which trades at an earnings multiple of more than 100.

Both stocks look investment-worthy given the opportunities ahead for each company in the weight-loss market, and investors should be careful not to assume Novo Nordisk is in trouble because it no longer has the hottest weight-loss drug available; it could still carve out a significant piece of the market for itself.