The FDA says over the counter sleep aids using acetaminophen in combination with diphenhydramine citrate failed to show any significant benefit. (iStock)

The U.S. Food and Drug Administration had data 15 years ago showing over-the-counter sleep aid products such as Tylenol PM and Excedrin PM don't offer a significant benefit to patients, according to a letter obtained by CBC News.

The products use a combination of the painkiller acetaminophen and diphenhydramine citrate, which is a sleep aid.

The letter was written by the director of the Office of Nonprescription Products at the Center for Drug Evaluation and Research at the FDA.

In the February 16, 2010, letter to the Consumer Healthcare Products Association, which represents the nonprescription drug industry, Dr. Charles Ganley rejects an industry sponsored study submitted to the Food and Drug Administration in 1995.

"FDA has reviewed the submission and concluded the study results do not demonstrate a contribution of both ingredients to the efficacy of the combination for OTC relief of occasional sleeplessness when associated with minor aches and pains," Ganley says in the letter.

According to the FDA, combination products such as the sleep aids must show a benefit over any single ingredient.

It's not clear why the FDA took 15 years to evaluate the industry's research.

Dr. Ganley's letter appears to be the first response from the FDA since the data was submitted on March 17, 1995.

The study was a multi-centre, randomized, placebo-controlled trial conducted in the early 1990s.

An analysis of the data suggests the combination products are statistically better than a placebo but not by much.

"We conclude that there is an insufficient basis to support the combination of acetaminophen and diphenhydramine as a nighttime sleep aid for the relief of occasional sleeplessness when associated with minor aches and pains," the letter goes on to tell drug makers.

The U.S. authority has not raised any concerns about the safety of the medications.

It has, however, invited the drug companies to conduct further research. The FDA says it needs two studies showing efficacy of the combination products.

The Consumer Healthcare Products Association has not returned calls from the CBC.