The world's largest study on sperm shape and size has identified cannabis as a major predictor of sperm abnormality.
Season and frequency of sexual activity were also major lifestyle factors affecting sperm but, surprisingly, cigarette and alcohol consumption had little effect.
The study included a sample of sperm from almost 2,000 men across the UK. The lifestyles of men with less than 4% normal sperm (defined as sperm with a typical shape and size) were compared against men with sperm samples with greater than 4% normality.
The University of Sheffield reported in a press release, "Men who produced ejaculates with less than 4% normal sperm were nearly twice as likely to have produced the sample in the summer months, or if they were younger than 30 years old, to have used cannabis in the three month period prior to ejaculation."
The researchers advise that men aiming to start a family should probably stop using cannabis as previous research has shown that good sperm morphology is crucial to successful conception.
The research was published in the journal Human Reproduction last week.