WASHINGTON — Shares of Monster Beverage Corp., which soared on Nov. 27 following the release of a letter from the Food and Drug Administration indicating an easing in concern over regulations for energy drinks, gave back some ground on Nov. 28 but still remained well above the 52-week low of $39.99 set on Nov. 8. Monster’s stock closed at $51.97 on Nov. 27 on the NASDAQ Global Select Market, up 13% from $45.88 on Nov. 26, but it was back down to $50.86 in late morning trading on Nov. 28.

The company’s share price spiked after the publication of a letter sent by the F.D.A. earlier this month to Senators Richard J. Durbin of Illinois and Richard Blumenthal of Connecticut.

In the letter, the F.D.A. acknowledged that energy drinks containing caffeine and other ingredients are a relatively new class of products, and although the products have the potential to raise safety or regulatory issues, there is a long history of safe use of other caffeine-containing products in the United States.

“F.D.A. is aware, however, that new products and patterns of use require us to remain vigilant, and we are working to strengthen our understanding of the nature of ‘energy drinks’ and any casual risks to health,” Michele Mital, acting associate commissioner for legislation at the F.D.A., wrote in a letter dated Nov. 21. “In particular, we are looking at whether products that may be safe for most individuals under labeled-use conditions may pose significant risks, arising from direct toxic effects, when the products are consumed in excess or by vulnerable groups, including young people and those with pre-existing cardiac or other conditions. This review includes investigating as fully as possible reported deaths and other serious adverse events that these reporting parties have associated with energy drinks.”

She said there is no scientific literature that calls into question the safety of taurine and guarana, two ingredients commonly used in energy drinks.

In terms of regulating caffeine levels in energy drinks, Ms. Mital said the F.D.A. is committed to taking science-based actions needed to protect public health, within the boundaries of its statutory authority and mandate.

“Depending on the outcome of our on-going review of the safety of ‘energy drinks,’ which includes caffeine alone and in combination with other ingredients, we will take action as needed with respect to the levels of caffeine in these products,” Ms. Mital said. “In addition, based on our safety review and within the bounds of our authority, F.D.A. will consider taking appropriate action with respect to the labeling of these products, such as requiring disclosure of the amount of caffeine in food products, limitations on intended use, or warnings about possible adverse effects.”

The F.D.A. said it may engage expertise outside the agency, most likely through consultation with the Institute of Medicine and possibly through an advisory committee or other public meeting.

The F.D.A. said it will investigate toxic effects when energy drink products are consumed in excess or by vulnerable groups, including young people and people with pre-existing cardiac or other conditions.

The American Academy of Pediatrics recommends no more than 100 mg of caffeine per day for adolescents. The F.D.A. letter cited a study on U.S. caffeine intake completed in September 2009 and revised in August 2010. It showed people age 14 to 21 consume a mean of about 100 mg of caffeine per day. According to the study, daily caffeine consumption for boys came from coffee (25 mg), teas (25 mg), carbonated beverages (45 mg) and other beverages (12 mg). For girls the daily caffeine consumption came from coffee (34 mg), teas (16 mg), carbonated beverages (36 mg) and other beverages (15 mg).

Monster’s share prices had tumbled recently amid concerns that regulators would crack down on energy drink makers after reports alleging that the highly-caffeinated products are tied to deaths came to light. But Monster has repeatedly said its drinks are safe and it does not know of any fatalities caused by its products. Monster said its drinks generally contain about 10 mgs of caffeine per oz.

Judy Hong, an analyst with Goldman Sachs, wrote in a research note that the F.D.A.’s response may signal that the agency’s study will not result in regulation that hurts Monster’s finances.

“At this point, the F.D.A. has little reason to think energy drinks are unsafe when used in a responsible manner,” Ms. Hong wrote in a Nov. 27 note.