It is "very unlikely" that anyone who was drinking alcohol until 2am or 3am the night before would be 100% sober the next morning, according to a leading medical expert.

Asked about reports that Taoiseach Brian Cowen did not leave the bar at the Ardilaun hotel until close to 3.30am last Monday night, Dr Eamonn Shanahan of the Irish College of General Practitioners (ICGP) said that, while he could not comment on an individual case, generally one unit of alcohol –or half a pint – takes an hour to leave the body.

"My considered opinion is that it is very unlikely, if you are drinking until late into the night before, that any individual would be 100% sober the next morning," he said. "If you were to do a blood alcohol measurement six hours after drinking say five pints, there would still be alcohol in your system.

"Broadly speaking, it takes around an hour for a unit of alcohol to leave your system. So, for example, four pints would take eight hours to clear from the system. This is something gardaí are aware of when they breathalyse people the morning after the night before, even if they have had some sleep. The alcohol is still in their system and they can be technically over the limit to drive."

A former chairman of the IGCP, Shanahan also said it was important not to overemphasise the role of a person's size or weight in mitigating the effects of alcohol the "morning after".

And while drinking water can help to lessen the dehydration that comes from alcohol intake, and perhaps slow the rate of consumption, he noted that this did little to speed up the rate at which alcohol would leave the body.

"Lack of sleep is also a significant factor. That is why we have the European working time directive for example," he said. "So it's a combination of both factors. If you are out until 3am and go on a radio programme at eight the next morning, having had a couple of pints before, you are certainly not going to be able to perform to the best of your ability. The point I would make is do one or the other – get plenty of rest or don't drink."

Dr Ronan Boland, chair of the GP committee with the Irish Medical Organisation, also would not comment on the specific case of Cowen's Morning Ireland interview. But he said another major consideration when it comes to the impact of alcohol consumption is its effect on a person's blood sugar level. This is lowered by alcohol consumption and can be helped by eating the next day.

"Low blood sugar and dehydration as a combination will influence one's ability to perform a variety of tasks the next day," he said. "If somebody has drunk seven or eight pints, the key thing is how long you give your body in terms of a break from this. If somebody is still drinking until two in the morning, and six hours later gets into their car, they will generally only have broken down three pints of alcohol."