For all their admonitions about responsible drinking, it turns out that older adults aren't as good as young ones about knowing when to stop. After drinking the same amount of alcohol as their younger counterparts, older adults are not only more impaired than younger ones, but less likely to believe they are.

In a new study published in this month's issue of the Journal of Studies on Alcohol and Drugs, researchers recruited 42 adults: half were between the ages of 50 and 74, and half were aged 25 to 35. The participants were given either alcoholic drinks — roughly equivalent to a couple of glasses of wine — or placebo beverages, then asked to perform tasks designed to test their motor skills. They were also asked to rate their level of intoxication on a scale of 1 to 10. While the older people were more impaired by the alcohol, they also consistently underestimated their drunkenness. That may be because over the years, people become inured to the effects of social drinking, says Dr. Sara Jo Nixon, the senior researcher on the paper. "It's a subtle effect," she says. "It's not one of those where you readily think, 'Oh my God, I shouldn't be driving.'"

And that's precisely the danger, says Nixon, a professor of psychiatry at the University of Florida who specializes in substance abuse. She and her colleagues wanted to study the effects of a relatively small amount of alcohol, the amount adults — mature adults, that is — might typically consume at dinner or in other social settings where drinking isn't the main event. Researchers tailored the composition of their cocktails — a mixture of medical-grade alcohol and limeade — to the participants' weight and gender, to achieve an average blood-alcohol content of .04%, half the legal driving limit in most states of .08%. Nixon was surprised by the results. "We often want to say that if we are below a legal limit, there are no consequences. That just isn't true," she says. Though older participants considered themselves only marginally impaired, on average, they were about five seconds slower in completing the motor-skills tasks. "That doesn't seem like much," Nixon says. "But it's an awfully long time if you're moving your foot from the gas to the brake."

Alcohol researchers parse the effects of intoxication — both on the ascending arm of the curve, in which people are on the road to drunkenness, and the descending arm, as the booze wears off. Generally speaking, on the upward slope, alcohol has a stimulating effect — "social lubrication," says Nixon. But on the down slope, as the alcohol is metabolized, its effects are more sedating. To measure the relationship between people's actual and perceived impairment along this continuum, researchers tested subjects twice, at 25 min. and 75 min. after they'd begun drinking. The two motor-skills tasks used to measure actual impairment were akin to connect-the-dots, but slightly more complicated. The first test simply required participants to draw lines between numbered dots in order; the second required them to alternate between numbers and letters in order as they connected the dots.

Compared with the 20- and 30-somethings, older adults believed they were less impaired as the alcohol's effects first hit; later, however, as they started sobering up, they perceived themselves as much more affected. "On the ascending limb, the [older] adults who got alcohol performed significantly more poorly, but they didn't think they were impaired," Nixon says. "On the descending limb, the older adults thought they were impaired, but at that point alcohol didn't have any impact on their performance." As to why the more seasoned social drinkers may be out of sync, Nixon says, "Older adults are mentally more sensitive to the sedative effect."

Prior to the study, all participants were carefully screened for health problems; those who had serious chronic medical conditions were ruled out, as were cigarette smokers, substance abusers and the extremely overweight. However, several participants in both age groups were taking over-the-counter or prescription medications for conditions ranging from depression to high blood pressure, a situation that accurately reflects the general population, says Nixon. Researchers deliberately chose "young" adults around 30 to make sure they were at least several years removed from the undergraduate binge-drinking culture. She notes that the less consistently people binge-drink, the more vulnerable they become to the effects, and after-effects, of alcohol (which is why you rarely hear a college kid complaining, "I can't drink like I used to"). "We didn't want that age group where there's really a lot of heavier drinking on a regular basis," Nixon says. "We wanted college graduates who had only histories of moderate drinking."

Past studies have suggested that people metabolize alcohol more slowly as they age and it takes them longer to clear alcohol from their system; alcohol may also alter brain chemistry differently in older folks. (That's why Nixon warns people against going out drinking with their parents. "You'll embarrass both of you," she says.) But the discrepancies in impairment between age groups in the current study were not attributable to differences in metabolism. Despite self-perceived differences in intoxication, actual increases in blood-alcohol content happened at similar rates in both age groups — which may be due in part to the fact that these older adults were healthier and more robust than average, says Nixon.

Nonetheless, says Nixon, older people should probably pay better attention to their own social-drinking habits. "Make sure you've given yourself adequate time to metabolize," she says. One drink per hour is probably a safe bet on average. "It's not bad advice for any age group, but it may be particularly fitting for older drinkers," says Nixon.

She also suggests that physicians and other health-care providers ask their patients more probing questions about alcohol consumption. Too often, she says, the issue is addressed perfunctorily: "Do you drink?" Check. "Socially?" Check. Then the doctor moves on. Instead, Nixon thinks providers should instigate a conversation, asking questions that raise self-awareness: Are you still having wine with dinner? How much? Why? "It doesn't make you 'old' to monitor your drinking," Nixon says. Just smart.