Novo Nordisk's (NVO 1.79%) Ozempic is so popular that it's practically self-marketing. For shareholders, that popularity has translated into achieving a total return of 48% over the past 12 months. And so it's natural for investors to wonder where to look for the next big thing so as to position themselves for the largest gains down the line.

While there's no predicting the future, right now there's enough evidence to make a case for the hottest biopharma field of tomorrow: longevity. Here's why.

The long-term case for investing in longevity

Getting older is a fact of life, but experiencing diminished health as a result isn't necessarily inevitable. There's no fountain of youth awaiting discovery, but it's reasonable to expect that in the coming years there will be medicines or interventions developed to help people live longer and healthier lives by addressing the biological impacts of aging.

Such developments are becoming known under the group heading of "longevity," and there are a handful of companies in the space already. A report by Allied Market Research says the global longevity market was worth $25 billion in 2020, and it could be worth as much as $44 billion by 2030.

There are a lot of different conditions (and investment opportunities) related to aging, depending on how broadly or narrowly you interpret the concept.

When it comes to developing treatments for age-related neurodegenerative conditions like Alzheimer's disease, Biogen already has a pair of products on the market, and competitors like Eli Lilly are in the process of getting other candidates approved. There are also marketed treatments for other common maladies related to aging like osteoporosis.

But addressing more-fundamental issues, like the process of cellular senescence (aging) itself, could be where the largest profits lie. In short, the cells in people's bodies become less vigorous as they age, which in turn is thought to cause a smattering of different aging-related problems.

Developing therapies that effectively target the core mechanisms of cellular aging could well make interventions for specific aging-related conditions obsolete. And it goes without saying that many millions of people -- or perhaps even billions -- would flock to take such a treatment if it existed, assuming it were safe.

The catch is that such medicines are, for now, still the stuff of science fiction. Though there are a slew of ongoing clinical trials investigating potential compounds with anti-aging properties in different disease contexts, there isn't yet much evidence of broadly applicable candidates.

So if you're itching to get into the longevity market early, it might be a bit too early to start.

This opportunity needs a while to cook

The longevity segment of the biopharma industry isn't about to yield an ambrosia for aging within the next 10 years as there are still key elements of the underlying science of aging that need to be answered.

Still, if the space follows the same path as other emerging medical fields have in the past, investors can expect to get a number of hints that portend the presence of investable opportunities. There are three phases of the field's development to understand.

In biopharma research and development (R&D), it's an iron rule that low-hanging fruit gets picked first. In practical terms, this means that medicines which treat specific maladies that are downstream of aging are going to continue being commercialized before medicines treating the underlying cellular causes of aging itself.

In the first phase, which we're already in, expect new therapies addressing Alzheimer's disease, liver scarring, macular degeneration (degrading eyesight), and similar conditions to be modestly to moderately profitable as they arrive over the next six years or so. Most of those conditions are already addressed somewhat by existing medicines, making blowout revenue unlikely. But as long as it's easier to develop incremental improvements than it is to break into a more challenging vertical, development efforts will continue with gusto.

The hint to look for in the near term is when clinical trials for the above illnesses start to fail because the newer candidates aren't able to prove their superiority to the existing medicines. That will mark the industry's transition to other targets under the umbrella of longevity, and it'll be a signal to look for the companies developing those other solutions.

The next phase may be characterized by treating multifaceted and generalized issues, like aging-related frailty, and it could occur sometime in the early 2030s or perhaps even earlier. There's currently nothing that's fully effective for addressing generalized age-related mobility issues.

In the final phase, expect businesses to say they've developed medicines that treat or perhaps cure aging itself at a cellular level, like we discussed earlier. It's unclear when such medicines will hit the market, if they ever do. Thus, the thing to look for when vetting investments is their resiliency against third-party scientific and medical skepticism.

For the companies that can actually deliver on what their clinical trial data says, substantial stock gains await.