THOUSAND PALMS, Calif. -

"If you’re a cannabis user and you’re trying for a baby ... stop."

This advice comes from Dr. Allan Pacey, senior lecturer in andrology at the University of Sheffield in the United Kingdom and lead author of a new study that suggests using marijuana could increase a man's risk of fertility problems.

The study, published in the journal Human Reproduction, looked at how a man's lifestyle affects his sperm morphology: the size and shape of sperm. Researchers collected data from 1,970 men who provided semen as part of a fertility assessment.

All of the lifestyle information was self-reported, and researchers made no attempt to confirm accuracy. Of those men, 318 produced abnormal sperm, where less than 4% of it was the correct size and shape (as defined by the World Health Organization). The remaining men's sperm had a higher percentage at a "normal" size and shape.

"Cannabis smoking was more common in those men who had sperm morphology less than 4%," Pacey said. "Cannabis affects one of the processes involved in determining size and shape. And we also know that the way cannabis is metabolized is different in fertile and infertile men."

The study found that men who had less than 4% normal sperm were typically under 30 years old, had used marijuana within three months of giving their sample and were twice as likely to have provided their sample during the summer.

Any of those factors could have influenced sperm morphology, but Pacey said "the only thing we found that was a risk that a man can do something about was cannabis."

The researchers did not set out to study cannabis; they were simply collecting data about men’s lifestyles to identify risks to fertility. They looked at a number of possibilities, including cigarettes, alcohol, recreational drug use, employment history, BMI, medical history and the type of underwear the men wore. The researchers concluded that none of these were factors.

A third of all infertility cases are linked to the male partner, according to theAmerican Society for Reproductive Medicine (PDF). The society says marijuana is associated with impaired sperm function and should not be used by men trying to conceive.

Society President Rebecca Sokol says the study confirms previous studies that found a possible but not proven link between abnormal semen and sperm function and the use of cannabis. But she warns that the study does not have enough cases to draw definite conclusions.

"The take-home lesson of the article is that clinicians should counsel their patients on the possible relationships between lifestyle factors, abnormal semen parameters and fertility outcomes," Sokol said. "This should include a discussion that the data are often inconclusive, but the motto 'everything in moderation' is a wise approach for the couple who is planning a pregnancy."

Another paper on the health consequences of cannabis was published this week in the New England Journal of Medicine. Dr. Nora Volkow, director of the National Institute on Drug Abuse, and a team of the institute's researchers prepared a paper detailing the risks based on the strongest scientific evidence currently available. According to the paper, they wanted to dispel "the popular notion that marijuana is a harmless pleasure" and does not need to be regulated.

The paper details what the research shows are the adverse effects of recreational use, including the risks of addiction. Approximately 9% of those who try marijuana will become addicted; one in six of those who start as teenagers and 25% to 50% of those who smoke daily become addicted.

The researchers also wrote about the harmful effects of cannabis use on brain development, especially in kids and teenagers. Preliminary research shows that adolescents who are early-onset smokers are slower at tasks, have lower IQs later in life and have an increased incidence of psychotic disorders.

Other problems associated with marijuana use, according to the paper, include impaired short-term memory and motor coordination, altered judgment, effects on school performance, a higher risk of motor-vehicle accidents and higher risk of cancer and other health issues like heart disease and stroke.

"There is a widespread and growing perception among not only youth, but the public in general, that marijuana is a relatively harmless drug, and it has been difficult marshaling science to correct this perception," Volkov said. "The science of marijuana is far from settled, and this has allowed advocates of various positions to cherry-pick evidence to support their particular stance."

The review also lists some of the potential therapeutic benefits of cannabis. Conditions and symptoms that may be helped by marijuana treatment include glaucoma, chronic pain, multiple sclerosis, epilepsy, nausea, inflammation and AIDS-related anorexia and wasting syndrome, according to the report.

Volkow and her fellow researchers fear that as governments begin to modify marijuana policy toward legalization, recreational use will increase, as will a host of negative health problems.

However, Mason Tvert, communications director at the Marijuana Policy Project, says the report by the National Institute on Drug abuse researchers is not an objective review of current scientific evidence.

The Marijuana Policy Project has worked to reform marijuana policies and laws since 1995 at both the federal and state level. It lobbies for legislation that would replace marijuana prohibition in favor of legal regulation. It provided much of the staff and funding in the push to legalize and regulate marijuana for adults 21 and older in Colorado in 2012, and its goal is to pass, by 2017, at least 10 more laws that would regulate cannabis like alcohol.

"NIDA has long been criticized for prioritizing politics over science," Tvert said. "They fail to acknowledge any of the well-known research that refutes, and in some cases completely debunks, their conclusions. This more closely resembles a poorly written college essay ... than it does an objective, evidence-based journal article. Every objective study on marijuana has concluded that it poses far less harm than alcohol to the consumer and to society."