With shares of AstraZeneca (AZN 0.19%) returning 55% over the last three years, easily topping the market's return of 29%, it's no surprise why investors might be curious about whether there's more upside in store.

But is now the best time to buy the stock, given its upcoming catalysts and recent financial performance? Let's analyze one argument for why this opportunity is still ripe right now, then look at a counterargument.

The case for buying AstraZeneca stock now

The bull thesis for investing in AstraZeneca is that it's making a lot of big bets to ensure that its future will be more profitable than its recent past (which wasn't half bad).

It has more than 120 programs in mid-to-late-stage clinical development, meaning that they could reach the market within a few years. In the first nine months of 2023 alone, it commercialized more than 20 programs, most of which were dedicated to expanding the indications of its already-approved medicines. With such a massive pipeline, AstraZeneca has enough leeway for its candidates to fail again and again without it ever fully running out of chances to take another swing. That's a major point in its favor given the inherent difficulty of developing new medicines.

Management is hard at work with business development deals to ensure that the pipeline remains chock-full of opportunities. It's making a licensing deal with a biotech called Eccogene to capture some of the highly lucrative obesity-therapies market with a program similar to Novo Nordisk's blockbuster drug Ozempic that's slated to enter phase 2 trials in 2024. The agreement will set it back $185 million upfront, with the possibility for nearly $2 billion in milestone payments if everything goes as planned.

Plus, the company is forging a new collaboration in the gene therapy space financed by a combination of equity investments in a company called Cellectis and future milestone payments that could lead to as many as 10 new medicines. It might pay out as much as $220 million for each of those candidates, plus royalties. So, it's clear that the potential for substantial growth is on the table.

As if that weren't enough, AstraZeneca is also launching a new digital-health business called Evinova, which will be focused on providing various services for planning and running clinical trials. It's already working with leading clinical-research organizations (CROs) and other clinical-service providers to market its offerings and accelerate the development process. Over the next decade and beyond, Evinova could well be yet another driver of revenue growth, and it could have the additional benefit of being inexpensive to operate relative to the sales it brings in.

The case for waiting or looking elsewhere

There are a couple of arguments against making an investment, each of which is likely to be a dealbreaker for a different type of investor.

One issue with AstraZeneca stock right now is its premium valuation. Its price-to-earnings (P/E) multiple is 34, which is far above the pharmaceutical industry's average P/E near 25. Bargain hunters should thus look elsewhere. That pricing suggests it should perform more like a growth stock than a pharma juggernaut.

But over the last 10 years, it hasn't actually posted growth stock-like gains despite a hot run of top-line growth over the past three years. Its quarterly revenue is up by just 66% since late 2013, reaching $11.5 billion, and its quarterly free cash flow (FCF) only rose by 32% in the same period to arrive at $2.2 billion. Wall Street analysts aren't anticipating that its annual sales will experience rapid expansion at any point over the next couple of fiscal years. So it looks like the recent sales performance is what's making the stock expensive even though that performance isn't expected to continue. If you're looking for big growth, this isn't it.

Then there's the biggest issue of all. Despite all of the promising programs in the pipeline, all of the ongoing business-development activity, and all of AstraZeneca's ambitions in digital health, its top line of more than $44 billion in 2022 means that it takes a tremendous amount of additional money flowing in to actually make its total sales budge by an appreciable amount. This is a mature pharma giant, not a biotech; it needs to be doing industrial-scale research and development (R&D) just to tread water. At its size, it's hard to expand any faster than it already is.

Therefore, there probably isn't a strong reason to buy this stock right this second. Of course, it could still be a decent long-term pick if you're shopping around for something that's relatively stable and unlikely to collapse overnight.