You get what you pay for. That old adage is true with cars, houses, and any other thing you might buy -- including stocks. When something is cheap, there's usually a good reason why.

Of course, a low share price doesn't necessarily mean a stock is cheap. A high share price doesn't necessarily mean a stock is expensive, either. In many cases, penny stocks (which today include any stock with a share price below $5) are actually valued unattractively. 

However, sometimes these low-priced stocks have tremendous growth prospects that have yet to be realized. There are quite a few penny stocks in the top 100 most widely held stocks owned by Robinhood investors. These stocks tend to be very risky but could pay off in a big way if they achieve their potential. Here are three popular Robinhood penny stocks worth watching.

Three stacks of pennies on top of a stock chart

Image source: Getty Images.

OrganiGram Holdings

Most of the Canadian cannabis stocks in the top 100 on Robinhood aren't penny stocks. OrganiGram Holdings (OGI 4.46%) is an exception, with its shares currently trading at around $2.50. The company once seemed to be the brightest rising star on the Canadian cannabis scene but has seen its luster dim over the last year or so.

OrganiGram has delivered plenty of bad news so far in 2021. Its second-quarter results were downright ugly. The company's gross revenue plunged 29% year over year and it reported another big net loss. Earlier this week, longtime OrganiGram CEO Greg Engel unexpectedly stepped down.

So why should investors keep their eyes on OrganiGram? The company is expanding its presence in the high-growth cannabis edibles market with the acquisition of The Edibles & Infusions Corporation. Its cannabis brands remain very popular in Canada, especially in the key Ontario market. The retail environment in Canada is improving as COVID-19 concerns fade and more stores open.

British American Tobacco recently bought a 19.9% stake in OrganiGram. The tobacco giant saw the potential for the small Canadian cannabis producer and chose to invest in it above all other alternatives. When a big player like BAT invests in a cannabis company, that company is one to watch.

Senseonics Holdings

Senseonics Holdings (SENS -4.16%) ranks as one of the few medical-device makers among the most popular Robinhood stocks. It's been a penny stock for nearly the entire time since the company went public, with its share price rising above $5 only one time briefly earlier this year.

The company markets a continuous glucose monitoring (CGM) device, Eversense. Unlike other popular CGM devices, Eversense is implantable and is usable for 90 days. Senseonics awaits U.S. Food and Drug Administration (FDA) approval for a 180-day sensor that should be even more appealing to individuals with diabetes.

Ascensia Diabetes Care (ADC), a leader in diabetes-care products, teamed up with Senseonics to market Eversense. With close to $1 billion in annual sales, ADC brings the financial strength and global sales team to the table that Senseonics needs to effectively commercialize its CGM device.

With ADC at its side, Senseonics projects that it can generate annual revenue of between $150 million and $200 million by 2025 compared to only around $15 million this year. That's the kind of growth potential that should put this penny stock on investors' radar screens.

Tonix Pharmaceuticals Holding

If I could put only one popular Robinhood penny stock on a watchlist, I'd go with Tonix Pharmaceuticals Holding (TNXP 5.40%). It's a clinical-stage biotech with some intriguing pipeline candidates.

The company is currently evaluating its lead candidate, TNX-102 SL, in a late-stage study as a potential treatment for fibromyalgia. Tonix has already reported positive results from another late-stage study of the experimental drug. It should announce interim results from this second phase 3 study in the third quarter of 2021, with top-line results on the way in Q4.

Tonix is targeting other indications for TNX-102 SL, as well, including sleep disturbance related to post-traumatic stress disorder. The drugmaker's pipeline also features another clinical-stage candidate, TNX-1300, which Tonix hopes to soon advance into phase 2 testing as a treatment for life-threatening cocaine intoxication.

Then there's what I refer to as Tonix's "lottery tickets" -- COVID-19 vaccine candidate TNX-1800 and experimental COVID-19 antiviral therapy TNX-3500. TNX-1800 is especially interesting because it has the potential to provide long-lasting immunity to the novel coronavirus with a single shot. The vaccine candidate hasn't moved into clinical testing yet. However, there's at least a possibility that it could eventually be a big winner for Tonix.