There are several healthcare companies with market caps in the hundreds of billions. However, there isn't one among this group that is approaching the $1 trillion mark just yet. There are four companies in that group though that have the potential to get there: Johnson & Johnson (JNJ 1.19%)UnitedHealth Group (UNH 6.50%), Novo Nordisk (NVO -0.37%), and Eli Lilly (LLY -0.93%).

Here's the case for why each one of these four companies is likely to be more valuable in the future and which one might be the first to be valued at $1 trillion.

1. UnitedHealth Group

Health insurer UnitedHealth Group has a market cap of $480 billion and is nearly halfway to $1 trillion already. It's probable that UnitedHealth Group can double in value, given the aging demographics in the U.S. and the growing need for healthcare coverage. According to estimates from the U.S. Census Bureau, by 2050, there will be nearly 84 million people who are 65 years and older -- almost double the number there were in 2012.

UnitedHealth Group has also been busy with acquisitions that will expand its reach, including LHC Group, which is a home health company that UnitedHealth acquired earlier this year. Over the past three years, UnitedHealth has grown its sales by more than 34%, from $240.3 billion to $322.1 billion. And with more than $20 billion in free cash flow in two of the past three years, this cash-rich business has the potential to get bigger with more acquisitions and investments in the future.

The stock is an investment that could potentially make you a millionaire, given its long-term potential. 

2. Johnson & Johnson

Johnson & Johnson's market cap is around $520 billion. It will get a little smaller as it spins off its consumer health business this year, but that should help the company focus more on growth opportunities in its pharmaceutical and medical device segments, making it a better buy in the long run. 

Although it's facing some headwinds as some key drugs lose exclusivity (including top seller Stelara), the company is confident it can grow its pharma business to $60 billion in revenue by 2025. Acquisitions could bolster its sales even further (pharmaceutical sales topped $52.6 billion in 2022). Meanwhile, in the medical device business, last year, the company acquired heart pump maker Abiomed for $16.6 billion, which will help it grow that segment.

The big question mark for Johnson & Johnson is how quickly the stock can grow and how long it may take for it to double in value. At just under $95 billion in sales last year, its top line has increased by 16% in three years. It's a good growth rate, but perhaps not fast enough for its valuation to rise as quickly as other growth stocks.

3. Novo Nordisk

Danish biotech company Novo Nordisk is smaller in market cap at approximately $360 billion. But the strong demand behind its obesity and diabetes drugs is what has me optimistic that the business has the potential to not only catch up to Johnson & Johnson and UnitedHealth Group but potentially reach the $1 trillion mark first.

Investors love a good growing business, and Novo Nordisk fits the bill; its sales totaled 177 billion Danish kroner ($26.6 billion) in 2022 and were up 26% from a year ago. And that's largely due to the growth of its obesity care sales, which doubled.

Wegovy (weight loss) and Ozempic (diabetes) have been popular drugs to help people lose weight, so much so that the company has struggled to keep up with demand. Novo Nordisk is working on expanding its capacity, which should, in turn, lead to more sales growth down the line. It recently announced that it was buying 104 acres in North Carolina to build a new production plant and help with that aim. 

The excitement behind Novo Nordisk is evident in the stock's 32% gains over the past year, as it has been the best-performing stock on this list. 

4. Eli Lilly

Eli Lilly's $350 market cap makes it the smallest healthcare stock on this list. But it has also been a good buy over the past year, with its share price jumping 18% despite the current bear market. And investors should keep an eye out for Eli Lilly because it has the potential to be one of the hottest healthcare stocks to own.

Its diabetes treatment Mounjaro demonstrated even better rates of weight loss than Wegovy (22% of body weight versus 15%), and it could be approved for that indication later this year. Should that happen, investors can expect the stock to soar as sales will likely be through the roof in the years ahead. Another potential catalyst for Eli Lilly is whether its Alzheimer's treatment, donanemab, obtains approval. It performed better in trials than Biogen's Aduhelm, which obtained accelerated FDA approval in 2021 but ultimately ended up being a disappointment.

Although Eli Lilly's sales of $28.5 billion were flat last year, there's plenty of reason to be optimistic about the future as Mounjaro alone could be a drug that some analysts believe could be the best-selling drug ever, with annual sales potentially hitting $25 billion and higher.

Which healthcare stock will get to $1 trillion first?

Investors love good growth stocks, and while Johnson & Johnson and UnitedHealth Group are behemoths within the healthcare industry, they're not likely going to be as fast-growing as Eli Lilly and Novo Nordisk will be. And between those two companies, Eli Lilly is the business that I think could deliver stronger returns. Should Mounjaro obtain approval, that could be the game changer that propels the stock to achieve higher gains. Although its valuation is the lowest of the stocks listed here, Eli Lilly is the stock I would expect to hit a $1 trillion market cap first.