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Although retirees are generally advised to avoid undue amounts of risk, younger investors hoping to beat the broader markets shouldn't necessarily shy away altogether from buying the occasional speculative equity. Tech giants like Apple and Microsoft, after all, were once speculative plays in their own right. With this in mind, our Foolish contributors think Shopify (NYSE:SHOP), Trevena (NASDAQ:TRVN), and PotashCorp (NYSE:POT) are worth a look by folks searching for high-risk, high-reward types of investing vehicles. Here's why.  

Shopify may be the next Amazon, but not tomorrow

Jamal Carnette, CFA: Among financial professionals, it's well known that the proportion of your investment portfolio allocated to risky stocks should have an inverse relationship to your expected holding period. This is a fancy way of saying if you're retired, it's advised to manage your equity risk by allocating less to equities and opting for stocks with lower levels of total risk, even if they have potentially higher projected returns.

One stock I'm bullish on for in the long run is Shopify. The company provides retailers with a plug-in digital platform, much like long-term success story Amazon. Unlike Amazon, though, Shopify focuses on improving the digital platform on the merchant's website instead of an aggregated marketplace. In the end, Shopify is a much friendlier merchant solution than Amazon, where a vendor has to compete with thousands of other vendors, potentially including Amazon itself.

Unlike many recent IPOs that shot up quickly to only reverse once enthusiasm ebbed, Spotify has continued to outperform the market. Shopify shares now trade hands at approximately $43, more than 150% more than its $17 IPO price in May 2015. Investors continue to plow money into the investment because the company has reported four straight quarters of year-over-year revenue growth in excess of 90%. Even better, the company's gross merchandise volume, or GMV, was 106% last quarter, which shows the company is growing by helping its merchants succeed and not simply by increased prices.

Still, it's not a perfect investment. The company reported a wider loss last quarter than the year before as costs of goods sold outpaced revenue growth. In the short run, this could be interpreted as increasing employees in anticipation of more growth, but it could impact results if growth slows. Additionally, Shopify would most likely be negatively affected in the event a larger competitor, like Amazon, decided to enter into this niche business. So, while I like Shopify for investors with long timeframes, I'd stay clear if I was a retiree.

Trevena is an ultra-high-risk play, but the payoff could be spectacular

George BudwellIf you're looking for a stock that could skyrocket soon, clinical-stage biotech Trevena needs to be on your radar. Trevena's value proposition centers on the injectable analgesic oliceridine that's presently in two late-stage studies for moderate-to-severe acute pain. The short story is that oliceridine is designed to essentially be a replacement for the go-to acute pain drug morphine. To achieve this lofty goal, Trevena is assessing the drug's ability to relieve pain quicker and with fewer side effects than morphine in the post-surgical setting.

The good news is that investors won't have to wait much longer for oliceridine's late-stage results, with both ongoing trials expected to produce top-line data in the first quarter of 2017. And if oliceridine hits its primary endpoints in both trials, Trevena plans on filing for the drug's regulatory approval with the FDA by mid-2017, putting a possible launch on track for early 2018.

Where Trevena's story gets particularly intriguing is in oliceridine's commercial opportunity compared to the company's tiny market cap of around $300 million. The acute pain market in the U.S., after all, is valued at over $10 billion, and there's a clear need for alternatives to morphine. Put simply, if oliceridine can capture even 10% of this huge market, Trevena's stock would almost certainly sport a radically higher valuation.

Of course, clinical-stage drugs like oliceridine are far from a sure bet, even after generating impressive mid-stage results. So, while Trevena's compelling risk-to-reward ratio may warrant a smallish position, it's probably not a good idea to back up the truck, so to speak.   

A good turnaround stock, but not without risks

Neha Chamaria: A turnaround play can occasionally prove to be a great bet for investors in the long run, but it is almost always a bad idea for retirees; especially if the company operates in a cyclical industry industry. Turnarounds don't always, well, turn themselves around and cyclicalality inherently makes earnings and dividend growth unpredictable. Case in point: Fertilizer giant PotashCorp. While an intriguing situation for venturesome investors, this is one stock that retires should emphatically avoid. Here's why.

Potash markets have been hit hard in the past couple of years as prices plunged following a massive supply glut. Unsurprisingly, PotashCorp stock is down almost 50% over this time period. Thankfully, potash prices appear to be bottoming-out even as PotashCorp prepares to merge with Agrium Inc.. This union will create the world's largest publicly traded fertilizer company. If the merger does goes through, the deal should reduce the volatility in PotashCorp's earnings from fertilizer price shifts thanks to Agrium's solid foothold in the relatively resilient retail side of agriculture (seeds and crop protection products). That's a promising growth catalyst -- something value investors may find interesting today, especially as PotashCorp stock is hovering close to its 52-week lows.

However, I still wouldn't suggest PotashCorp to retirees as a potential investment for the simple reason that it is a commodity stock. A recovery in fertilizer markets could be painfully slow, limiting the growth in PotashCorp's earnings and cash flows. That could also mean unstable dividends: PotashCorp has already slashed its dividends twice this year. That's not to say I'm suggesting PotashCorp will cut its dividend again, but while young investors have time on their side and can take risks, retirees need stability -- something PotashCorp isn't capable of giving them.

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.