About a decade ago, patients started coming to Dr. Alexander Pastuszak with a question that has only become more common since then: Would smoking weed make it more difficult for them to have children?

Dr. Pastuszak, an expert in male fertility but not a cannabis user, knew that laws and norms around marijuana were being relaxed across the country, and that many men were comfortable discussing their drug use with their physicians. He just didn’t have anything concrete to tell them.

“Until recently, the answer was, ‘I don’t know,’ ” said Dr. Pastuszak, who is now a urologist at the University of Utah.

So he started looking for an answer. Cannabis is legal for either medicinal or recreational purposes in all but 11 states and was used at least once by 52.5 million Americans in 2021, according to the Centers for Disease Control and Prevention. Yet because it remains illegal under federal law, it’s hard to study, and its long term health effects are still poorly understood, said Dr. Omer Raheem, a urologist at the Cleveland Clinic who practices in the United States and Abu Dhabi.

Even so, when it comes to the male reproductive system, some effects of cannabis are becoming clear. By looking at 48 studies from around the world, Dr. Pastuszak found in 2019 that cannabis products do harm a man’s chances of conceiving children. Finally, he feels, he has an answer for his patients.

“THC, certainly in smoked form, can impact semen,” Dr. Pastuszak said, and therefore damage male fertility.