Marijuana use is becoming increasingly popular among youth in secondary school, particularly males. The recreational drug has also been under a lot of scrutiny amid its legalization for adult recreational use in Colorado and Washington, and medicinal use in 21 states. So, it's no surprise that more research on the drug continues to surface. British researchers suggest if you want to start a family, you should best lay off the schedule I substance drug. According to a study published in the journal Human Reproduction, smoking marijuana can affect male fertility by altering sperm size and shape.

"Our knowledge of factors that influence sperm size and shape is very limited, yet faced with a diagnosis of poor sperm morphology, many men are concerned to try and identify any factors in their lifestyle that could be causing this,” said Dr. Allan Pacey, lead author of the study and senior lecturer in andrology at the University of Sheffield, in the press release. Previous research has suggested that only sperm with good sperm morphology — normal-shaped sperm and volume of fluid — are able to pass into the woman’s body following sex to make their way to the egg and fertilize it. Sperm with an abnormal shape and size usually tend to swim less because they are not as efficient as their regular counterparts.

Lifestyle factors are suspected to affect sperm morphology, but there is not concrete evidence to validate this assumption. In an effort to examine what lifestyle factors are associated with poor sperm morphology, Pacey and his team of researchers analyzed data from a cohort of men in the UK. A total of 2,249 men from 14 fertility clinics were asked to fill out detailed questionnaires about their medical history and their lifestyle. The lifestyle factors examined included: body mass index (BMI), manual work, boxer shorts, alcohol, cigarettes, street drugs, cannabis, season, and abstinence.

The participants had previously provided a semen sample as part of infertility investigations with their partner. This sample was used by the researchers in this study. The men were asked to abstain from ejaculation for a period of three to five days. All of the information about lifestyle and health factors was self-reported, with no attempt made to confirm the accuracy of health reports, such as report of a fever or pelvic ultrasound. Events such as surgery to the testes, believed to indicate an irreversible risk, were included in the analysis if they occurred prior to the semen sample.

The findings revealed men who produced ejaculates with less than four percent normal sperm, smooth, oval-shaped heads, and no fluid droplets bigger than one half of the sperm head size were nearly twice as likely to have this occur in the summer months from June to August. In addition, participants who were younger than age 30 and used cannabis three months prior to ejaculation, also were found to have poor sperm morphology. Unlike marijuana, other lifestyle factors such as smoking cigarettes and drinking alcohol had little effect on sperm morphology.

“This research builds on our study of two years ago which looked at the risk factors associated with the number of swimming sperm (motile concentration) in men's ejaculates,” said Dr. Andrew Povey from the University of Manchester's Institute of Population Health in the press release.

Those men who produced a sample after six days' abstinence were less likely to be affected by poor sperm morphology. However, the researchers could not explain this finding and believe there is little evidence that delaying fertility treatment to make adjustments to a man’s lifestyle will improve his chances of conception. Pacey does advise, “It is therefore reassuring to find that there are very few identifiable risks, although our data suggests that cannabis users might be advised to stop using the drug if they are planning to try and start a family."

While there is no clear research that shows a direct relationship between tobacco, alcohol, or caffeine use, ReproductiveFacts.org says these substances may hurt sperm DNA quality.

Source: Pacey AA et al. Modifiable and non-modifiable risk factors for poor sperm morphology. Human Reproduction. 2014.