This has been a great year for Eli Lilly and Company's (NYSE:LLY) shareholders, but AbbVie Inc. (NYSE:ABBV) stock has actually lost ground in 2018. Lilly's shares have surged around 26% in 2018 but that doesn't make it the better stock to buy right now.

To figure out which of these pharmaceutical stocks has the best chance of outperforming the market in the years ahead we'll need to look past their recent gains. Let's figure out what's driving growth for both underlying businesses to see which of these pharmaceutical stocks is most likely to pull ahead in the long run.

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The case for AbbVie Inc.

The house that Humira built is quickly becoming an impenetrable compound defended by a growing product portfolio. That's critical because Humira's main U.S. patent expired last year. Despite its age, AbbVie's flagship rheumatoid arthritis (RA) drug is expected to rack up $20 billion in sales this year, and it could keep climbing. 

Less expensive biosimilar versions of Humira probably won't hit the market until 2023 and by then several of AbbVie's more recent drug launches could help offset the losses. This summer, AbbVie launched Orilissa, the first new treatment for endometriosis pain women have seen in over a decade, and it's expected to generate more than $1 billion annually within a few short years.

AbbVie also has results in hand that show its next RA drug, upadacitinib, could do even better if the Food and Drug Administration approves an application the company plans to submit later this year. During the Select-Early trial with newly diagnosed patients, those given 30 mg upadacitinib tablets were twice as likely to report RA symptoms that improved by 50% or better than those given methotrexate.

If upadacitinib can take methotrexate's place as a standard treatment for new RA patients, annual sales of the drug could reach $6.5 billion at its peak. That would go a long way toward raising the company's dividend payout, which offers a big 4.1% yield at the moment.

The case for Eli Lilly and Company

Eli Lilly's next-generation migraine drug, Emgality, recently earned an approval from the FDA to help prevent debilitating headaches that affect an estimated 30 million adults in the U.S. alone. Initial sales will ramp up slowly as the company absorbs heavy discounts in order to build a brand. Two competing therapies from the same class have already launched, but if Emgality can carve out a reasonable share of the space, it could generate several billion in peak annual revenue. 

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Emgality will join a new product lineup that already includes eight drugs launched since 2014. Lilly's young guns grew sales by a combined 73% during the first half of 2018 compared to the previous year period.

Lilly's developing another potential blockbuster for the treatment of arthritis pain in partnership with Pfizer called tanezumab that recently succeeded in a pivotal trial. Patients with osteoarthritis of the knee or hip, a large population in need of new treatment options, reported significant improvements in pain and physical function following two injections spread eight weeks apart. Millions suffer from osteoarthritis pain, which means a successful new treatment option could go a long way toward raising Lilly's dividend payouts.

At recent prices, Lilly shares offer a 2.1% dividend yield that's above the market average but far less than AbbVie stock offers.

Running the numbers

Over the past three years, AbbVie's raised its dividend payout 88% higher, and investors can probably look forward to more big bumps in the years ahead. The company needed just 44% of free cash flow to meet its dividend obligations in the past year, leaving plenty of room for further increases. Eli Lilly shelled out 89% of free cash flow to make payments over the same time frame, so don't expect the next raise to be a significant one. 

Right now, Lilly shares are priced for growth at 19.8 times this year's earnings expectations, while AbbVie shares can be picked up at the low price of just 12.0 times expected 2018 earnings. As long as Humira sales don't plummet ahead of 2023, it looks like AbbVie shareholders will come out ahead in the long run. That makes it the better buy right now.