Over the past decade, Eli Lilly (NYSE: LLY) has produced amazing returns, becoming the largest healthcare stock in the world in the process -- and the first to hit a $1 trillion valuation. Lilly may not be able to maintain the pace it has set over the past 10 years, but it still has enough fuel to help turn patient investors into millionaires by the time they retire. Here's why.

An innovative powerhouse

Lilly has performed well in recent years thanks to major breakthroughs, especially in diabetes and weight management. And although this area is the drugmaker's main growth driver right now -- and will remain so well into the 2030s -- it is actively looking to diversify its lineup. The company is notably making a push into the large oncology field.

Will AI create the world's first trillionaire? Our team just released a report on the one little-known company, called an "Indispensable Monopoly" providing the critical technology Nvidia and Intel both need. Continue »

Patent cliffs for its current main products won't come any time soon, but diversification will help Lilly navigate such losses when they do. Beyond any specific drug it is developing, it is also investing heavily in technology, including artificial intelligence (AI). The company is partnering with Nvidia to build the most powerful AI supercomputer in the pharmaceutical industry, as well as a research lab where its experts will work with AI engineers.

Management's goal is to accelerate the discovery and development of breakthrough therapies. These initiatives won't pay off immediately; they might not even make much of a dent in the next five years.

But they are helping set a solid foundation for the future, and that's one thing long-term investors should focus on. Yes, Eli Lilly is posting excellent financial results right now, riding the wave of its best-selling drug, tirzepatide, approved for diabetes and weight management.

The company isn't just sitting on its laurels, though. It is using that success and investing wisely in ways that will pay off a decade from now and beyond.

Don't put all your eggs in one basket

Eli Lilly does seem to have many of the qualities required to help make investors millionaires, given enough time -- say, a few decades: Besides a solid business and an innovative culture, there's its excellent dividend program -- it has more than doubled its payouts in the past five years alone. Reinvesting the dividend will significantly boost total returns over the long run.

However, the key word here is "help," as in, it likely can't do it all by itself. Investors should ensure they have a relatively large, well-diversified portfolio. Eli Lilly can reasonably be one of the core holdings in such a portfolio, helping investors retire as happy multimillionaires.