$Eli Lilly and Co (LLY.US)$ enjoyed meteoric success with its weight loss drugs Mounjaro and Zepbound — the latter already recording stellar sales only three months after launch in December.

Recent data showed that, for the first time since launch, Zepbound filled more U.S. prescriptions in a single week than rival $Novo-Nordisk A/S (NVO.US)$'s Wegovy.

Whether or not this becomes a trend remains to be seen in the coming weeks, but it illustrated the power of Eli Lilly's product portfolio.

And its weight loss drugs weren't even the company's fastest-growing treatments in terms of sales. Sales of its cancer drug Verzenio, according to full-year results, grew 56% year-on-year in 2023 to record $3.8 billion in sales.

Weight-Loss Market Set For Growth

But it's the battleground for diabetes and weight-loss treatments that captured the interest of investors, and it's a market that is expected to grow.

According to the International Diabetes Federation, around 10.5% of the global adult population had diabetes in 2021, and it predicts that by 2045 nearly 800 million adults will be living with the condition as growing urbanization, aging population and decreasing levels of physical activity increase the number of overweight and obese people.

According to the World Obesity Federation 2.7bn people, or 38% of those aged older than five, are obese or overweight.

Last year Barclays estimated the total weight-loss drugs market could be worth $200 billion by 2030, while Bloomberg sees annual sales of $80 billion by 2030.

Eli Lilly is expected to generate sales of nearly $3 billion for Zepbound in its first year.

Duopoly To Be Challenged

Together Eli Lilly and Novo Nordisk currently represent an almost duopoly in the weight-loss treatment market and demand for these treatments is already rising so quickly they are struggling to meet it.

Both companies have engaged in deals with manufacturers to help produce the so-called "skinny pens" that deliver patients a weekly dose of the treatment.

And these successes have been very noticeable in the share prices. Eli Lilly stock is up 32% year-to-date, and up 132% over the past year, rising to 318% taken over a three-year time scale.

Eli Lilly's stock price gain has taken its market cap up to a staggering $734.3 billion, ranking it as the world's most valuable pharmaceuticals company, and the ninth-biggest public company in the world.

If investors think these gains put the stock in the realms of bubble territory, remember the potential markets for its drugs as the world's birth rate slows and the population becomes increasingly older.

Don't forget, however, that the market won't remain a duopoly for long. There are already reported be around 70 new treatments in various stages of development and trial. Such is the potential for the market.