According to a new study of the causes of male infertility — the largest of its kind to date — smoking marijuana is even more likely than drinking alcohol and smoking cigarettes to cause sperm to develop abnormally. Researchers recently published their findings in the medical journal Human Reproduction.
The research team, led by Allan Pacey, MD, a senior lecturer in andrology at the University of Sheffield in England, recruited more than 2,000 men from 14 fertility clinics in England and gave them detailed questionnaires about their medical histories and lifestyle habits. When cross-analyzing the sperm samples with the information on the forms, researchers found that men with abnormal sperm samples were twice as likely to have ejaculated during summer months, and they were much more likely to have abnormal sperm when they had smoked cannabis in the three months prior to ejaculating.
For a point of reference, researchers used guidelines published by the World Health Organization which detail appearance and exact dimensions of normal sperm. It's thought that sperm with abnormal morphology (meaning its size and shape) swim poorly and will have more difficulty reaching a woman's egg to fertilize it.
Dr. Pacey says that studies done on laboratory animals suggest that marijuana can affect the way the DNA in sperm is packaged together. "The cannabis seems to be interrupting the DNA folding properly," he says.
Additionally, the doctor noted that it usually takes up to three months to produce a sperm, so men hoping to conceive a baby with their partners may want to abstain from marijuana use for an extended period of time.