Marijuana has been reported to damage male fertility. This is not good news for those who smoke it. Marijuana is now the most popular recreational drug in the developed world, and is being made legal in more and more countries. So this link between marijuana and male fertility is worrying. The most recent research shows that amongst a group of young men, those who smoke marijuana more frequently have poor quality semen.
This is made more significant by the fact that it backs up previous work so the story seems to be true! The results show clearly that marijuana decreases sperm count (~30%), and goes on to show that if using marijuana alongside another recreational drug, your chances of conception drop even further! It was found that smoking marijuana doesn’t have a linear effect upon semen quality the more it’s used, but instead a minimum amount is required for it to have a harmful effect on fertility.
The previous study which this work was based on showed that when a different form of the active ingredient of cannabis was studied in an animal model, sperm production as well as sperm movement was lower. This shows that smoking marijuana may have a bad effect on fertility, and that your chances of starting a family can be affected by your lifestyle.

For more information see here:

http://www.conceivingbyart.com/marijuana-and-male-fertility/