Mid-cap stocks sometimes get overlooked because they are often past the exciting early days that lure growth investors but aren't yet large enough to garner widespread investor attention. Investors can simply miss the long-term opportunities these types of stocks offer because they are frequently provided very little Wall Street coverage.

That's a shame in many cases, because mid-cap stocks can still have plenty of growth ahead of them. Misunderstood drugmaker Amarin (AMRN -1.38%), taser maker turned software-as-a-service company Axon Enterprise (AXON 2.62%), and precious-metals-focused Royal Gold (RGLD -0.41%) are all great examples of mid-cap stocks that deserve a little extra investor attention right now. Here's a primer from three Motley Fool contributors.

A mid-cap biopharma with room to run

George Budwell (Amarin): Mid-cap biopharma stocks can be outstanding growth vehicles. These companies usually have at least one product on the market and therefore have passed through the eye of the needle in terms of the enormous risk posed by key clinical-trial readouts.

A woman with dollar signs flowing from her fingers and onto a bar chart that is heading higher

Image source: Getty Images.

Amarin, for instance, is a mid-cap biopharma play that has a Food and Drug Administration-approved omega-3 treatment, Vascepa, on the market. The drug's sales are set to kick into hyperdrive soon due a positive readout from its large cardiovascular-outcomes trial known as REDUCE-IT. The best part of this story, however, is that the market has yet to fully price in Vascepa's commercial opportunity as an add-on to statin therapy in patients at risk of cardiovascular disease. 

Why is the market doubting Vascepa's commercial potential? Skeptics have knocked the drug because the mineral oil placebo used in REDUCE-IT may not have been totally inert. So the claim is that Vascepa's 25% relative risk reduction of serious cardiovascular events observed in this trial might be overstated.

But even an incremental decrease in Vascepa's cardioprotective benefit -- stemming from this debatable placebo issue -- shouldn't be enough to derail a label expansion for this high-value indication. Vascepa fills an important gap in care for cardiovascular disease patients, and it's hard to argue otherwise based on the currently available data.  

Where is this stock headed? With sales forecast to top at least $2 billion in the next decade, Amarin's shares should have no problem pushing past their current range, once Vascepa's label is officially expanded early next year (assuming no unexpected hiccups with the FDA). It may take some time for the market to fully recognize Vascepa's value proposition, but this growth story definitely has legs.  

Big releases of new services are on the horizon

Brian Stoffel (Axon Enterprise): I wouldn't blame anyone who follows my writing to think I'm a little obsessed with Axon. I have repeatedly called it out as one of the best mid-cap stocks to buy. But there's a reason for that: I'm fully convinced the company's new slate of services will spur huge growth in the decade ahead.

For those who are unfamiliar, Axon used to be known as Taser International. The company still makes its stun guns for police departments. But the name change occurred to highlight a new growth driver: police body cameras.

What has investors excited is how these cameras created an opening for Axon to become a software-as-a-service (SaaS) company. Departments need somewhere to store and analyze all of their footage. Axon's Evidence.com acts as a cloud-based subscription solution for that need. Already, the company has over 300,000 police officers registered as users on Evidence.com.

But later this year, Axon will be taking it a step further with the release of Axon Records. The service will take the audio and visual cues from body cameras, apply artificial intelligence and machine learning, and auto-fill the paperwork that keeps cops trapped behind a desk -- instead of out in the community. 

Because of special deals Axon is giving away to get departments into the Axon Records ecosystem, it will take time for sales growth to appear. But with just a $4 billion valuation, I think there's tons of room for Axon's stock to grow in the years ahead.

Add something hard to your portfolio

Reuben Gregg Brewer (Royal Gold): The market is once again near all-time highs, and volatility appears to be picking up. For proof, look no further than the roughly 2.6% drop in the S&P 500 Index on May 13 based on continuing trade tensions with China. But here's an interesting fact: The shares of Royal Gold were up 1.4% that day.

RGLD Chart

RGLD data by YCharts.

Royal Gold is a precious metals streaming company. To simplify things, that means it provides miners with up-front cash (used to build mines, for expansions, or for debt reduction) in exchange for the right to buy precious metals at reduced rates in the future. It's a fairly low-risk model in the precious metals space that locks in wide margins since Royal Gold's costs are predetermined. Yes, its stock will fluctuate along with precious metals, but that's actually the point here.

Royal Gold is a great way to add exposure to hard assets, often viewed as a safe-haven investment. You avoid the complications of owning mining assets, get the upside from growing production (that is, after all, what Royal Gold's investments largely help to fund), and in the case of Royal Gold, benefit from a steadily rising dividend. The dividend yield is modest, at just 1.2%, but the payment has been increased annually for 18 years. Meanwhile, Royal Gold continues to grow, adding a new stream earlier this year. It also has around $800 million of capacity to make additional deals as large miners look increasingly willing to streamline by selling mines to smaller players. 

In other words, if you buy Royal Gold today, you get exposure to a diversifying asset (precious metals), a growing dividend, and growth potential. It's hard to complain about that.