The more alcohol men consume, the weaker the quality of their sperm, a study has found.

Drinking just five units of alcohol every week could reduce the quality of a man's sperm, new research suggests.

And the more alcohol consumed, the weaker the quality of the sperm, the study found.

Men of a reproductive age should be advised to steer clear of habitual drinking, the authors said.

The study, published in the journal BMJ Open, examined 1200 Danish male military recruits between the ages of 18 and 28, all of whom underwent a medical examination between 2008 and 2012.

As part of the assessment they were asked about their drinking habits and invited to provide sperm and blood samples.

The average number of units drunk in the preceding week was 11.

The researchers found drinking alcohol in the previous week was linked to changes in reproductive hormone levels, with the effects increasingly noticeable the more alcohol was consumed.

Testosterone levels rose while sex hormone binding globulin (SHBG) fell, they found.

Almost half said the quantity of alcohol drunk in the preceding week was typical of their normal weekly consumption.

Among this group of men, researchers found that the higher the tally of weekly alcohol units, the lower was the sperm quality in terms of sperm count and the proportion of sperm that were of normal size and shape.

The effects were evident in those who drank five or more units a week and most apparent in those who drank 25 units or more.

Those who typically drank 40 units a week had 33 per cent lower sperm count and also had less "normal looking" sperm than men who drank between one and five units a week.

"Our study suggests that even modest habitual alcohol consumption of more than 5 units per week had adverse effects on semen quality although most pronounced associations were seen in men who consumed more than 25 units per week," the authors wrote.

Copyright AAP 2014