Investing in healthcare can be an excellent way for investors to diversify their portfolios and take a position in strong, stable markets that will continue to see demand for the foreseeable future. The two stocks listed below not only have a lot of long-term potential but have already delivered strong results this year with both of their share prices doubling, dwarfing the S&P 500's returns of 29% in 2019.

1. Seattle Genetics

Seattle Genetics (NASDAQ:SGEN) was having a good year in 2019 with the stock up around 30% in September. Things really started to take off for Seattle Genetics when the company released results of its phase 1 clinical trial for EV-103, which is designed to treat patients with urothelial cancer, the most common type of bladder cancer. The company announced that "the study met outcome measures for safety and exhibited encouraging clinical activity for this platinum-free combination in a first-line setting." It's a good first step for the drug, but phase 1 is still very early on in a drug's testing and it is no guarantee that EV-103 will hit the market anytime soon. It could still be years before all the necessary testing is complete.

The company got another boost in its share price in October when it announced positive results from a trial evaluating its tucatinib drug in treating patients with HER2-positive breast cancer. The drug was used with trastuzumab and capecitabine and showed encouraging results, so much so that the company announced it would submit a new drug application in early 2020. 

People working in a lab.

Image source: Getty Images.

On Oct. 29 the company released its third-quarter results for 2019, which showed strong year-over-year sales growth of 26%. And although losses rose from $67.4 million last year to $91.9 million in Q3, given the company's progress in its drug trials in recent months, it's easy to see why investors haven't been discouraged by the results. 

There's a lot of potential for Seattle Genetics to rise even higher if the company continues to see progress from the development of its drugs.

2. Insulet

Insulet's (NASDAQ:PODD) stock has fallen in December, but the stock has still had an impressive year in 2019. Strong Q2 results in early August gave it a big 21% jump in price as it recorded a profit and raised its guidance for revenue for the remainder of the year. The medical device company's Omnipod Insulin Management System saw strong sales growth outside the U.S. with international Omnipod revenue rising 120% from the prior year. 

The stock also surged in November when it released strong Q3 results and again raised its guidance. And the company still has a lot of potential, as Insulet announced in December that patients in Europe will begin using the Omnipod Dash Insulin Management System. The new system "provides up to three days of non-stop insulin delivery, without the need for daily injections and allows individuals to discreetly and intuitively manage their diabetes via a new touchscreen interface allowing for greater freedom and flexibility to focus on living life to the fullest." The U.S. Food and Drug Administration approved the new system and it could provide people with diabetes a more discreet, convenient way of managing their insulin. 

Which stock is the better buy today?

Both Seattle Genetics and Insulet have significant growth potential over the long term. But neither stock is cheap today. Insulet trades at a massive 70 times its book value and 15 times sales; Seattle Genetics, meanwhile, trades at 11 times its book value and 24 times its top line. However, these numbers can change quickly as these companies continue to evolve and grow their top and bottom lines. 

From a risk standpoint, the safer healthcare stock today is Insulet. Although it's pricey, given its growth and that its products are already generating strong results, it's much more of a sure thing. Seattle Genetics could produce stronger results than Insulet if its drugs are successful in further trials, but that's by no means a guarantee.

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.