Just because a big pharma company is successful today doesn't mean it will be successful in a few years. Even the biggest blockbuster drugs eventually lose patent exclusivity or face new and improved competition. That's why continued research and development (R&D) is a must. 

Throwing money at R&D doesn't guarantee success -- but not making smart investments in developing pipeline candidates will almost certainly ensure failure. The odds for success are better for drugmakers that can invest in plenty of experimental drugs. 

Which big pharma stocks should investors own based on R&D spending levels? Here's why Roche (RHHBY 0.10%), Novartis (NVS -0.32%), Pfizer (PFE -1.70%), Johnson & Johnson (JNJ 0.04%), and Merck (MRK -0.15%) rank at the top of the list. 

View from below of an assortment of pills and a man holding pill in gloved hand.

Image source: Getty Images.

1. Roche

Roche invested $8.7 billion in pharmaceutical research and development last year, the most of any big pharma company. The figure represents roughly 22% of Roche's total prescription drug sales in 2016. Market research firm EvaluatePharma projects that the Swiss healthcare company will retain its No. 1 spot in 2022.

What will Roche's R&D spending produce in the coming years? Two of the biggest winners are likely to be emicizumab and lampalizumab. Emicizumab is being evaluated in late-stage studies for treatment of hemophilia A, while lampalizumab is being tested in a late-stage study targeting treatment of geographic atrophy associated with age-related macular degeneration. Roche expects to file for regulatory approval for both drugs in 2018. 

2. Novartis

Switzerland is also home to the second-biggest R&D spender among big pharma. Novartis invested $7.9 billion in pharmaceutical research and development in 2016, comprising a little over 19% of total pharmaceutical sales. EvaluatePharma also thinks the company will keep its ranking for R&D spending in 2022.

One especially promising pipeline candidate for Novartis is its experimental CAR-T cell therapy CTL-019. In July, an FDA advisory committee unanimously recommended approval for the drug in treating B-cell acute lymphoblastic leukemia (ALL). Novartis also expects to file for regulatory approval for five new molecules in 2018.

3. Pfizer

The world's leader in prescription drug sales, Pfizer, ranks third in R&D investments. In 2016, Pfizer spent $7.8 billion on R&D, roughly 17% of total pharma sales. However, EvaluatePharma projects that Pfizer will slip a spot in its ranking for both prescription drug sales and R&D spending by 2022.

Pfizer's pipeline includes 32 late-stage programs. The company also awaits regulatory approval for several more candidates. Two of Pfizer's most promising pipeline assets are breast cancer drug talazoparib (picked up in the 2016 acquisition of Medivation) and diabetes drug ertugliflozin, which Pfizer is co-developing with Merck. 

4. Johnson & Johnson

Johnson & Johnson takes the No. 4 ranking, with $7 billion in R&D spending in 2016. That reflects 22% of the healthcare giant's total pharmaceutical sales. EvaluatePharma predicts that J&J will move up to third in R&D spending by 2022.

J&J's pipeline made EvaluatePharma's top five as well. One potential big winner from that pipeline recently won FDA approval -- plaque psoriasis drug Tremfya (guselkumab). Johnson & Johnson also has high hopes for apalutamide, which stands out as one of the most exciting cancer drugs in late-stage development.

5. Merck

Merck invested $6.8 billion in R&D last year. This level represents a little over 19% of the company's total pharmaceutical revenue. EvaluatePharma thinks that Merck will hold on to the No. 5 spot in 2022.

Although Keytruda is already a huge winner for Merck, it also makes up a big part of the drugmaker's pipeline. Multiple studies are underway for Keytruda, either as a monotherapy or in combination with other drugs in treating various types of cancer. Merck also could have another blockbuster on its hands if its collaboration with Pfizer on diabetes drug ertugliflozin pays off with regulatory approvals.