Drinking just five units of alcohol every week could reduce the quality of a man's sperm, new research suggests.
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 1,200 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.
Testosterone levels rose while sex hormone binding globulin (SHBG) fell, they found.
Those who typically drank 40 units a week had 33pc 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 five 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.
"Young men should be advised to avoid habitual alcohol intake," they added.