Judging ‘morning after’ driving condition an uncertain science

Several variables, including pace of drinking, weight and gender, affect alcohol absorption

That more drivers over the alcohol limit "the morning after the night before" are being apprehending by An Garda Síochána is not surprising. A combination of late-night binge-drinking followed by an early start means people are often driving with higher blood alcohol concentrations than they realise.

The rate of absorption of alcohol depends on several factors: gender, weight, pace of consumption, type of drink taken and alcohol concentration are just some of the variables.

Absorption is quickest when alcohol is drunk on an empty stomach and the concentration of alcohol is 20-30 per cent.

Gastric emptying

For example sherry, with an alcohol concentration of about 20 per cent, increases the levels of alcohol in blood more rapidly than beer, while spirits, at 40 per cent, delay gastric emptying and inhibit absorption. Drinks taken with carbonated mixers get into the system more quickly.


Food, particularly carbohydrates, delays absorption. On an empty stomach, blood alcohol concentration peaks about one hour after consumption, depending on the amount drunk. It then decreases in a more or less linear manner for the next four hours.

Alcohol is generally removed from the blood at a rate of about 15mg per 100ml per hour but this varies in different people and with the amount of alcohol drunk.

Normal functioning

All of which means enough alcohol to impair normal functioning could be present the morning after an evening session of drinking three pints of beer or three double whiskeys.

The legal limit for blood alcohol while driving in is 50mg/100ml. However, blood alcohol concentrations as low as 30mg/100ml are associated with an increased risk of accidental injury. At 80mg/100ml, the risk of a road traffic incident more than doubles.

The other element of safe driving is how quickly alcohol is eliminated from the body. About 95 per cent of ingested alcohol is eliminated by the liver with most of the rest excreted via sweat, urine and the breath.

But the health of your liver, whether you are taking medication and other factors can significantly change this rate.

Because there are so many variables, attempting to work out how soon after a drinking session you are under the driving limit is an uncertain science. Far better to follow the practice of commercial pilots: no alcohol for a minimum of eight hours “bottle to throttle”.

Muiris Houston

Dr Muiris Houston

Dr Muiris Houston is medical journalist, health analyst and Irish Times contributor