Getting rich quickly by investing in stocks is theoretically possible but extremely difficult. Instead, a buy-and-hold strategy might be the best bet for growing your capital. This method allows for the magic of time to put your money to work. That's why some of the most celebrated investors have a long-term focus when picking companies in which to invest.

But which stocks are truly worth holding for decades? Let's look at two excellent candidates: Eli Lilly (NYSE:LLY) and Pfizer (NYSE:PFE). If you have $5,000 available that you aren't keeping for a rainy day, investing at least part of that money in one or both of these drugmakers -- and holding on to them a decade or more -- would likely be a great move. Let's see why.

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1. Eli Lilly

A longtime leader in the diabetes drug market, Eli Lilly has a history that dates back well over 100 years. Its historical performance compares favorably to that of the broader market, and there are good reasons to believe the healthcare giant can keep things up for many years to come.

For one thing, Eli Lilly still has a solid lineup of diabetes medicines. The company's top-selling product is Trulicity, a non-insulin diabetes drug. Sales of Trulicity jumped 45% year over year in the third quarter to $1.6 billion. Sales of Jardiance, another diabetes medicine, increased by 26% year over year to $390 million.

Some of Eli Lilly's non-diabetes products are performing well, too. Verzenio, a cancer medicine, recorded $335 million in revenue, 43% higher than a year ago. And Taltz, an immunosuppressant, racked up $593 million in sales, 30% higher than the prior-year quarter.

Eli Lilly is also benefiting from sales of its antibodies, including bamlanivimab, which earned Emergency Use Authorization (EUA) a year ago for treatment of COVID-19. In the third quarter, coronavirus-related sales were $217 million while total revenue jumped 18% year over year to $6.8 billion. Not including its COVID business, revenue still increased by 11%, a very good performance.

Smiling pharmacist.

Image source: Getty Images.

Meanwhile, Eli Lilly is looking for more growth avenues. It recently initiated a rolling submission to the U.S. Food and Drug Administration (FDA) for accelerated approval of donanemab, a potential medicine for Alzheimer's disease. Elsewhere, it is working on a potentially game-changing, once-a-week insulin product called Basal Insulin-Fc. That's just the tip of the iceberg for the company as it has several dozen programs in its pipeline.

One issue with Eli Lilly is valuation. The company's forward price-to-earnings (P/E) ratio of 33 is much higher than the average forward P/E of about 14 for the pharma industry. As a result, there could be some uncertainty and volatility in the short run, but for those with a long-term mindset, Lilly is an excellent pharma stock.

2. Pfizer

Over the past year, Pfizer has been on fire thanks to its coronavirus vaccine, Comirnaty, which it developed with BioNTech. This product has helped Pfizer's top and bottom lines grow by leaps and bounds.

Still, there remain some cracks in the company's business. Perhaps the biggest issue Pfizer currently faces is the regulatory scrutiny over Xeljanz, an immunosuppressant that belongs to a class of drugs known as JAK inhibitors. In September, the FDA started requiring JAK inhibitors to carry a warning of possible severe adverse reactions (including a higher risk of cardiovascular events and cancer) after Xeljanz was shown to carry these risks in a post-marketing study.

Sales of the drug have been harmed as a result. In the third quarter, Xeljanz's revenue was $610 million, a 7% decrease compared to the third quarter of 2020. That said, this headwind is still a small issue in a large and otherwise solid business. Pfizer's total revenue for the third quarter, not including sales of its COVID vaccine Comirnaty, grew 7% year over year to $11.1 billion.

That good performance was helped by such products as anticoagulant Eliquis, which saw sales for the period jump 21% to $1.3 billion. Other top performers included cancer medicine Inlyta, which grew sales 31% to $256 million. Meanwhile, Pfizer's biosimilar business recorded revenue of $575 million, 36% higher than the year-ago period. And we can't ignore Comirnaty, which had sales of roughly $13 billion for the quarter.

Doctor writing on a clipboard.

Image source: Getty Images.

When including revenue from its coronavirus vaccine, Pfizer's top line skyrocketed by a whopping 134% year over year to $24.1 billion. The company now expects $36 billion in revenue from this vaccine in 2021, which is even higher than its previous guidance of $33.5 billion. Comirnaty continues to rack up regulatory approvals, including in children age 5 through 11, and in vaccinated adults who desire a booster dose.

Sales of Comirnaty could drop substantially once the pandemic subsides, but Pfizer has accumulated loads of cash thanks to it. The company currently has roughly $29 billion in free cash flow, which could allow it potentially to acquire promising pipeline programs from smaller companies. That would supplement its already-full pipeline, including 29 phase-3 studies. For instance, it has a promising COVID pill in the works called Paxlovid.

Pfizer's future is bright, and with a reasonable forward P/E of about 12, the company looks like a strong buy right now. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.