A couple of stocks that have more than doubled this year are Nano-X Imaging (NNOX 0.45%) and C3.ai (AI 3.02%). But as lofty as their valuations have gotten, they are still nowhere near the highs they hit in 2021. Can these stocks get back to those levels again, or is it too late to invest in these businesses?

1. Nano-X Imaging

Up roughly 100% so far in 2023, Nano-X has been one of the best healthcare stocks to own this year. The company doesn't generate a whole lot of revenue ($8.6 million over the trailing 12 months) and it remains unprofitable. But what investors are bullish about is its future, with the company's multisource X-ray system, Nano.ARC, recently obtaining 510(k) safety clearance from the Food and Drug Administration.

Nano-X says the system can make medical imaging both more affordable and accessible. The uncertainty is how much revenue it can generate because Nano-X plans to deploy it on a pay-per-scan basis. That means if there isn't robust demand and hospitals and clinics aren't using it, sales might not be that strong. 

As well as the stock has performed this year, at about $15 it's still nowhere near the high of $94.81 it hit on Jan. 27, 2021. That surge took place when meme stocks and risky investments were hot buys, including Nano-X. Valuations got out of control, and it's fair to say that its shares probably weren't worth such an inflated price tag. Unless a similar phenomenon were to occur, it may not get back to those levels anytime soon.

At more than 80 times revenue and nearly 4 times book value, Nano-X's stock is already trading at a pretty sizable premium. The odds of it getting back to its 2021 high seem unlikely. But investors can still earn a good return from the stock if the company is able to demonstrate demand and revenue growth from its X-ray system.

Until that happens, however, I'd hold off on buying shares because Nano-X has been a volatile investment, averaging a beta of 1.5, which indicates that it can move wildly when compared to the broader stock market.

Although it may not be too late to buy this stock, which has a market cap of around $800 million, there's still a bit too much risk to make this a tenable investment right now. Investors are better off taking a wait-and-see approach with Nano-X.

2. C3.ai

A business that's further along in its growth path than Nano-X is C3.ai. The tech company has benefited from the skyrocketing interest in artificial intelligence (AI) this year, soaring more than 260% since January. Its ticker is AI and that's also what its business centers around, offering AI-related solutions for multiple industries. 

At about $41, the stock is trading at just a fraction of the $176.94 high it hit on Feb. 10, 2021. That was also during the meme hype but it was before AI was the focal point for many investors. These days, companies are rolling out new AI-backed products and services as they look to leverage new technologies and chatbots. 

This is another case where I'm optimistic C3.ai's stock can rise in value but I'm not confident it can get back to its 2021 highs only because investors are more conservative now. At 16 times revenue and nearly 5 times its book value, its valuation is by no means cheap. But investors could become more bullish on the business because its revenue growth could accelerate in the future. This fiscal year (which ends in April), the company projects that its revenue will be between $295 million and $320 million, which would translate into growth of at least 10.6%, better than the 5.6% increase it achieved last fiscal year.

However, even that might be a low target. The company's chief executive officer, Tom Siebel, recently told Barron's that the business is in a good position to "beat-and-raise" its estimates.If that does indeed happen, it wouldn't be too surprising to see the stock continue to rally this year. But as with Nano-X, investors need to be careful; this also is a volatile investment, with C3.ai's beta also being fairly high at around 1.4.

If you can tolerate the risk (C3.ai is unprofitable), this can still be a good stock to own, particularly if you're bullish on AI.