Since the prescription sleep aid medication Ambien first arrived on the market in 1993, it’s become a favorite among insomniacs, shift workers, and jet-lagged travelers. But there’s a catch - the sleeping pill comes with a potentially dangerous drug dose gender gap. The U.S. Food and Drug Administration (FDA) discovered during a blood test study that men metabolized the drug much fast than women. This led the FDA to cut recommended doses for women by half.

Shortly after, in 2014, the findings prompted the FDA to release a set of guidelines to the pharmaceutical industry detailing the exceptions for excluding a gender from clinical trials, such as testing a prostate cancer medication or other male or female-specific drug. Because women have only been included equally in drug studies since 2001, there are still many pharmaceuticals circulating on and off the shelves at the local drug store, many of which still haven’t been tested on women.

Recently, the FDA lowered the recommended doses for Ambien and the extended-release version Ambien CR. Too many people who were prescribed the drug were complaining of high levels of impairment the morning after, which made for a dangerous combination when they had to drive or perform an activity that required mental alertness. Women, in particular, appeared to be at the greatest risk. They’re more susceptible to the effects of Ambien because their bodies process it slower than men, in a similar way alcohol’s effects are delayed and linger longer after their first drink.

Ambien falls into the drug category of hypnotics because it’s designed to fight off sleepless nights on a short-term basis by tricking the brain. It works by first activating the neurotransmitter (brain cell messenger) GABA and binding it to a receptor that slows down the brain within 20 minutes. It effectively knocks you out, but hits women twice as hard. Although Ambien works in the brain, it’s metabolized in the liver. Since women have lower concentrations of liver enzymes than men do, the drug processes through them at a slower rate, putting them at risk for a leftover morning effect.

Historically, women of childbearing age had been routinely excluded from drug trials for decades. Women have also been excluded from sports research because fluctuating levels of estrogen and progesterone during menstrual cycles made it different to keep consistent clinical investigations.

“This is not just about Ambien — that’s just the tip of the iceberg,” Dr. Janine Clayton, director for the Office of Research on Women’s Health at the National Institutes of Health, told The New York Times. “There are a lot of sex differences for a lot of drugs, some of which are well known and some that are not well recognized.”

It was only until the FDA discovered aspirin affected women differently when it came to lowering the risk of heart disease and stroke that the agency decided to start testing on both men and women. Yet there still remains a wide and ambiguous gap, leaving women to hope that they will react to a particular drug or prescription the same way men did in the clinical trials.