Image source: Getty Images.

Many investors prefer large-cap stocks, as the underlying companies tend to be mature and stable. However, it's a mistake to overlook mid-cap stocks, which are generally defined as having a market capitalization of between $2 billion and $10 billion. Companies that fit this mold tend to be on firm financial footing, but their smaller size can mean they have greater upside potential.

With that in mind, here are three mid-cap stocks from the healthcare sector that look like buys today.

Growth potential to smile about

While the name Align Technologies (ALGN -0.75%) may not ring a bell, my hunch is that you've heard of the company's Invisalign clear aligners. This product gives anyone with malocclusion -- or misaligned teeth -- an "invisible" way to fix their smile instead of traditional braces. Invisalign also allows patients a few other advantages beyond the cosmetic appeal, too: Users can eat whatever food they want, and the device can be removed at any time.

Image source: Align Technologies.

Unsurprisingly, the company's system has proven to be hugely popular. Thanks to years of double-digit growth, Align Technologies has turned into a highly profitable company that's currently valued at over $7 billion dollars. Thankfully, I don't think investors have missed the boat just yet, as recent results suggest that this company's growth story is still going strong.

Last quarter Align put up top-line growth of 29%, which juiced the bottom line by an even stronger 59%. The company's system appears to be a hit in international markets too, with sales up a strong 35% over the year-ago period. Analysts believe the company will be able to sustain that growth rate for quite some time, too: Current projections call for earnings per share growth of more than 23% annually over the next five years.

Of course, Wall Street is aware of the company's growth projections and has priced shares accordingly. Right now Align's stock sports a trailing P/E of 45, which is quite pricey. However, with such high growth coming in the years ahead, I still think investors should considering nibbling on this stock today and then coming back for more when the share price takes a hit.

Sounds like a strong investment

Picture this: A company that has grown profits by more than 20% over the past five years is projected to grow them by more than 18% over the next five years -- and it trades for less than 11 times forward earnings. Sound too good to be true? It's not, as those numbers perfectly sum up the current investing thesis for Jazz Pharmaceuticals (JAZZ -1.36%), a commercial-stage biotech currently valued at $7.5 billion.

Jazz's golden goose is a drug called Xyrem, which treats the rare sleep disorder narcolepsy. This blockbuster drug is projected to grow by double digits this year and ring up more than a billion dollars in annual sales. Combining that with the rest of the company's product portfolio, Jazz currently projects full-year non-GAAP EPS of $9.90 to $10.30.

Sounds great, right? Well, the market doesn't think so, as that range is actually down from its prior outlook of $11.10 to $11.50 in EPS. Thankfully, Jazz has a good reason for the revised guidance. 

A few months back, Jazz shelled out $1.5 billion to acquire Celator Pharmaceuticals and thereby get its hands on the company's lead compound, Vyxeos. This drug produced impressive clinical results in treating high-risk acute myeloid leukemia, or AML, a cancer of the blood and bone marrow. In fact, regulators gave it both breakthrough therapy and fast-track designations.  

Of course, the acquisition is temporarily inflating the company's cost structure, squeezing profitability at the moment. However, Jazz believes the deal will be accretive to earnings by 2018, and sales of Vyxeos could grow to as much as $900 million by 2020. If so, then that will nicely balance out Xyrem sales and propel revenue and profits higher for years to come. I think that makes Jazz a great candidate for new capital right now.

Image source: Getty Images.

A market all to itself

With a market cap of just under $4 billion, the final company on our list is the smallest and riskiest of the group, but it could offer the greatest upside potential.

The company in question is Acadia Pharmaceuticals (ACAD -0.40%), a biotech that has only just recently reached the commercial stage. In April, Acadia won approval for its first drug, Nuplazid, which is approved to treat Parkinson's disease psychosis, or PDP. This disease afflicts roughly 400,000 Americans and causes them to experience hallucinations and delusions. 

Caring for patients with PDP is incredibly difficult, which often forces the patients into nursing homes. Not only is that highly disruptive to the patient's life, but it is also expensive. Since Nuplazid is the first and only approved drug to help treat this condition, the market potential looks huge.

Acadia assigned Nuplazid an annual wholesale price of $23,400, so if patient and provider demand for this drug lives up to expectations, then Nuplazid could easily turn into a blockbuster medication. That's especially true if the company can also introduce Nuplazid into foreign markets and expand its labeling to include conditions like schizophrenia and Alzheimer's disease psychosis. 

If Nuplazid can deliver on its expectations, then Acadia's sales could be set to explode, quickly turning the company into a profitable enterprise. If so, then its stock could hold a lot of upside from today's prices, so risk-loving investors might want to give this company a closer look.