This article was originally published on Nov. 20, 2016, and was updated on August 5, 2017, to reflect the latest clinical and commercial updates for each company. 

Penny stocks can be simply defined as stocks that trade at less than $5 per share. Of course, there are other criteria such as trade volume and market cap that some will check before labeling something as an official penny stock, but for the purposes of this article, we'll stick to just share price. These "cheap" stocks are typically exciting but far too risky to be worthy of any serious consideration by traditional investors. However, the biotech space is arguably one intriguing exception to this general rule of thumb. 

Biotech stocks, after all, can fall hard and fast on a single clinical or regulatory setback. And on rare occasions, these beaten-down stocks can mount monumental comebacks, creating jaw-dropping gains in the process. 

With this in mind, let's take a deeper look at why Amarin Corp. (AMRN -0.53%) and Geron Corp. (GERN 2.29%) may be worth buying right now, while MannKind Corp. (MNKD 0.72%) and Novavax (NVAX -0.95%) should probably be avoided. 

Scientist in white coat and safety goggles looking at flasks in a lab setting

Image source: Getty Images.

Amarin is gearing up for a healthy 2017

Amarin has been growing by leaps and bounds lately because of the blockbuster potential of its highly refined fish oil pill Vascepa. Although Vascepa's sales over the last year only came in at around $130 million, its sales trajectory could make a hockey-stick-like move soon.

The keystone issue is that Vascepa's large cardiovascular outcomes trial known as REDUCE-IT is tracking as predicted and should produce top-line results in early 2018. Now, the big ticket item here is that REDUCE-IT could provide the first empirical evidence that an omega3-based therapy does lower the risk of serious cardiovascular events in patients with high triglycerides who are already taking statins.

If that proves true, Vascepa's sales should jump from the hundreds of millions into the multibillion-dollar range, based on the enormous size of the lipid-lowering drug market. Beyond this sizable payoff, though, Amarin's stock is a compelling buy simply because the risk of failure with REDUCE-IT is arguably baked in to a large degree. Vascepa's sales, after all, won't go to zero in a worst-case scenario, and Amarin's shares certainly aren't trading at a premium right now.

Geron could skyrocket in 2017

Cancer treatment specialist Geron's value proposition is all about its novel telomerase inhibitor imetelstat, which is being developed by Johnson & Johnson through its biotech subsidiary Janssen Pharmaceuticals. Although J&J's second internal review of imetelstat's ongoing trials for the blood-based disorders myelofibrosis (MF) and myelodysplastic syndromes (MDS) yielded encouraging results, the latest clinical update that hit the Street at the end of July 2017 caused investors to panic.

In short, J&J decided to refine the target patient population for the drug's MDS indication, which could curtail its commercial opportunity in a big way. Additionally, J&J said it would let the drug's mid-stage MF data mature further before pulling the trigger on a continuation decision for this, or any, indication. In other words, imetelstat doesn't appear to be producing a compelling efficacy profile in either MF or MDS at this stage. 

On the bright side, imetelstat does sport blockbuster sales potential as a novel blood cancer treatment, and J&J does have a knack at guiding promising anti-cancer agents through the bumpy clinical trial process.   

The big "catch" with Geron -- and perhaps a good reason not to buy this speculative stock -- is that the company is entirely dependent on imetelstat's fate. If this drug goes belly up (and that's starting to look like a distinct possibility), Geron probably will as well -- which is almost certainly the underlying reason why this stock is trading at a tiny fraction of imetelstat's commercial potential at the moment. 

MannKind and Novavax have razor-thin margins of error right now

MannKind is simply too risky to own because of the low probability that the sales of its inhaled insulin product Afrezza will surge high enough for the company to avoid bankruptcy. Even after Afrezza's former marketing partner, Sanofi, decided to hand MannKind a couple of major gifts toward the end of 2016, and the company subsequently underwent a major managerial shake-up in 2017, there are still serious doubts that this struggling biotech can keep its doors open all that much longer. To do so, MannKind will either have to find a new partner or somehow figure out a way to get Afrezza's commercial launch back on track. Neither of those scenarios is likely, unfortunately. 

Novavax, for its part, is still reeling from the stinging negative result of its late-stage respiratory syncytial virus (RSV) vaccine candidate in elderly patients last year. To keep its doors open and live to fight another day, the biotech decided to slash its workforce by 30% late last year and attempted to resurrect its RSV vaccine by launching another mid-stage trial in older adults. To be fair, Novavax could pull a rabbit out of its hat with a win in its late-stage RSV trial for maternal immunization, but that possibility looks like an extreme long shot based on its modest mid-stage results and the vaccine's complete wipe out in its trial for elderly patients.

The moral of the story is that share price means relatively little on its own. Some "cheap" penny stocks may be great speculative buys, but many more are a trap best avoided.