Monster Beverage (MNST) investors are getting jittery, and not for their usual highly caffeinated reasons.

The company's shares have lost more than a quarter of their value in the past two days after the Food and Drug Administration confirmed Monday that it had received "adverse incident reports" of five deaths involving Monster energy drinks. The New York Times first drew attention to the reports, which were requested under a Freedom of Information Act filed by the mother of a 14-year-old Maryland girl who died in December from heart problems after drinking Monster Energy on two consecutive days.

A lawsuit filed by Anais Fournier's parents in California Superior Court on Oct. 17 alleges the girl drank two Monster Energy beverages that contained a combined 480 milligrams of caffeine, equivalent to about 21 8-oz. servings of Coca-Cola (KO) or 19 8-oz. servings of Pepsi (PEP). According to Bloomberg, the Maryland medical examiner’s office determined the girl’s cause of death to be from "cardiac arrhythmia due to caffeine toxicity."

The FDA says it hasn't found a link between Monster Beverage drinks and the reported deaths, but is still investigating.

Though Monster Beverage issued a statement last Friday saying it was was "saddened" and vowing to vigorously defend itself, investors don't seem comfortable waiting around for the result. The company's share price dropped 14% on Monday and more than 11% on Tuesday despite Monster's strongest rebuttals.

"Monster does not believe that its beverages are in any way responsible for the death of Ms. Fournier," the company said. "Monster is unaware of any fatality anywhere that has been caused by its drinks."

This likely isn't going to be an isolated jolt for Monster, either. The company is named in five adverse incident reports to the FDA this year and 37 overall since 2004. That's making the FDA reconsider its stance on energy drink caffeine limits, which could also spell trouble for Red Bull, 5-Hour Energy and other "energy" products.

The FDA already has restrictions in place for soda, and limits caffeine content to 71 milligrams per 12 ounces to be considered safe. The 24-ounce Monster drinks cited in the lawsuit contain almost seven times that much caffeine, but exploit a loophole in the FDA's caffeine guidelines, labeling themselves "diet supplements," to avoid the limit.

They're often found in the same convenience store coolers and grocery aisles as soda, but Monster relies heavily on its labels to let folks know the drinks are "not recommended for children and people who are sensitive to caffeine."

Funny, since researchers at Johns Hopkins University have been calling for FDA-mandated labeling of energy drinks since 2008. This fight is seldom solved with a label, though. Anheuser-Busch InBev (BUD) and MolsonCoors (TAP) each pulled caffeine-spiked alcoholic energy drinks from the market in response to lawsuits. The makers of the alcoholic energy drink Four Loko were only able to overcome a scolding from the FDA bans in several states by pulling caffeine, taurine and guarana out of its formula and taking "energy drink" out of its description and title.

The accusations against Monster are far more severe and could result in major changes to the energy-drink industry regardless of outcome. Investors rarely enjoy being part of such test cases and may not stick around to see how this one turns out.

More from Top Stocks

• DuPont layoffs reflect broader job market woes
  
• The fiscal cliff has arrived
  
• Stop bogarting tobacco stocks