After a dismal performance in 2018, Aurora Cannabis (NYSE:ACB) is rocking so far this year. Shares of the Canadian marijuana producer have soared by nearly 40% year to date. In January, Aurora announced the acquisition of Whistler Medical Marijuana, which has earned a reputation for its high-quality cannabis products.  

But is everything really so rosy for Aurora right now? Not necessarily. Here are three of the biggest worries investors should have about Aurora Cannabis.

Marijuana plants in a greenhouse

Image source: Getty Images.

1. Mediocre international sales growth

Aurora Cannabis Chief Corporate Officer Cam Battley stated last year that "not everyone has fully appreciated" the potential for the global medical marijuana market. The company plays up the fact that it's active across five continents and 22 countries, with a leading market share in Europe and Latin America. Aurora's management team believes the global medical marijuana market could reach around $70 billion Canadian, which translates to more than US$50 billion.

But despite all this, international sales still barely move the needle for Aurora Cannabis. Even worse, the company's international sales growth is only mediocre. Aurora reported its second-quarter results on Feb. 11. International sales accounted for only 6% of total revenue. And those sales increased by only 1.8% from the previous quarter.

Granted, it's still early for most international medical marijuana markets. But Germany, which is the biggest market in Europe, legalized medical cannabis nearly two years ago. Battley said in Aurora's Q2 conference call that Aurora Cannabis is "a medical company at heart." The company will need significantly greater international medical marijuana sales in the future to prevent shareholders from experiencing heartburn. 

2. Slowness to move into the U.S. hemp market

The U.S. legalized hemp in December, and Aurora's top rival, Canopy Growth (NYSE:CGC), quickly announced its intention to enter the U.S. market. Less than a month later, Canopy followed up with an update stating that it had secured a hemp license in New York state and planned to invest more than $100 million to build a large-scale hemp production facility.

Aurora has been vocal about in the past about its desire to enter the U.S. market. The company is no stranger to hemp production, with several subsidiaries that focus on hemp. And the U.S. hemp market could be as large as $22 billion by 2022, according to projections from the Brightfield Group.

With all of this in mind, you might think Aurora Cannabis would be hot on Canopy's heels in entering the United States. Nope. When asked about the company's plans in the Q2 conference call, CEO Terry Booth replied that Aurora would "enter when it's proper to enter, and when it's legal to enter into the United States market." 

For Canopy Growth, the time to enter the U.S. already appears to be both proper and legal. Aurora Cannabis investors should be concerned about the company's slowness to expand into a potentially massive new market.

3. No big partner on the horizon

Two of Aurora's chief rivals, Canopy Growth and Cronos Group, have landed important deals with significant equity investments by big companies outside the cannabis industry. A couple of other Canadian marijuana producers have also partnered with large alcoholic-beverage companies but without equity investments.

Aurora Cannabis, however, has been left out of the party so far. Sure, there have been rumors of deals. Last September, there were even stories circulating that Coca-Cola (NYSE: KO) was in discussions with Aurora. But nothing happened.

Securing a deal with a major partner isn't just a status symbol. Constellation Brands' $4 billion investment has given Canopy Growth a huge cash stockpile to fund expansion -- including, as previously mentioned, into the United States. The longer Aurora goes without a big partner -- and the big dollars that come along as a result -- the more worried investors should be about the company's ability to go toe-to-toe with Canopy on a global scale.

Worry not?

It could simply be a matter of time before all three of these worries subside.

As international medical marijuana markets mature, Aurora's sales in those markets will increase. The company might be slow to jump into the U.S. hemp market, but perhaps the early cautious approach could pay off over the long run. And if global cannabis sales begin to grow as much as many expect they will, it's likely that big partners from outside the cannabis industry will come calling on Aurora.

However, Aurora's lofty market cap assumes rapid growth for the company. If one or more of these big concerns aren't resolved, Aurora's share price could plunge -- making the stock's great start in 2019 only a temporary hurrah.