Marijuana and low testosterone: the two go hand in hand. Yes, marijuana is a "herb". Yes, marijuana is a "natural plant". However, none of that is going to change the fact that marijuana is a one way ticket to low testosterone, erectile dysfunction and fertility problems. The political debate regarding marijuana may be interesting, but the research shows it to be debilitating and potentially deadly long term.
Marijuana assaults your precious hormones in almost every negative way possible, especially testosterone. One study after another has shown that cannabis lowers testosterone. For example one research team found that "a reanalysis of existing data established that testosterone levels are depressed both after smoking one marijuana cigarette and after intravenous infusion of delta-9-tetrahydrocannabinol, a pharmacologically active component of marijuana".  The same study concluded that it would take at least 24 hours for testosterone levels to normalize after marijuana use. (NOTE: It's not just the smoke - an IV will do it.)
Another study found that not only was testosterone decreased after short term marijuana use, but leutenizing and follicle stimulating hormone were lowered as well.  And just to add to the endocrinological misery, the arch-villain and stress hormone cortisol was raised as well in this same study. There are also studies in animals and humans that strongly indicate that marijuana blunts growth hormone response as well.
But that's where the good news ends: animal studies show that marijuana use shrinks the testes.  So, if you're not happy with lowered testosterone, infertility and elevated cortisol, you can sit around enjoying the fact that you've got a little more air flow through your boxers. (You may also be interested in my pages on Other Common Causes of Low Testosterone and How to Increase Your Testosterone Naturally for more information.)
You should also know that there are many reports that chronic marijuana use leads to gynecomastia, i.e. "bitch tits" or "male boobs" due to its abundant amounts of phytoestrogens. One journal writer pointed out that "given the effects of marijuana on the HPG axis in males and the possibility that noncannabinoid components of marijuana smoke have affinity to the estrogen receptor, an association with gynecomastia is plausible but has not been convincingly demonstrated".  Remember that estrogen fights against testeosterone in the body as well.
Marijuana has also recently been flagged as particularly dangerous for young people because it decreases seratonin and increases norepineprine. While these are not sex hormones like testosterone, these can alter mood negatively and, through prolonged use, may permanently alter anxiety levels and reaction to stress.  Again, the researchers are suggesting this may have long term, possibly lifetime anxiety and mood repercussions. I would also add that any increase in stress will also likely lower testosterone as well.
So we ask the question, "Could someone please explain again why anyone in their right mind would smoke marijuana?" The only thing we can think of is the extra hydrogen cyanide. That's right - marijuana tobaco is much higher in hydrogen cyanide - probably five times higher - than cigarette tobacco. Maybe that partially explains why habitual pot smoking is so hard on the lungs and why cannabis use has also now been linked to the most aggressive form of testicular cancer.
I am sure most of you have the good sense to stay away from the wacky weed and have heard every excuse in the book from its adherents. I have heard one guy claim it helps him stay cut because it suppresses appetite, not realizing that it is whacking his precious muscle-building testosterone behind the scenes. To those sorts of arguments, one can only give them a look that relays the idea that sticking burning leaves in your mouth and inhaling is never going to be a smart idea! (Other toxic items that we consume include Excitotoxins, Pesticides and Additives Such as BPA.)
Not to make the bad news even worse, but there is also considerable reported evidence of erectile dysfunction among chronic marijuana users. This is undoubtedly partially due to the lowered testosterone. However, the other reason was discovered by one study that showed marijauna effected Nitric Oxide and summarized by saying, "We conclude that early endothelial damage may be induced by chronic cannabis use (and endocannabinoid system activation".  In other words, the happy weed may not make you so happy long term: it may take your sex life with it.
The tragedy with marijuana is that many cultures and youth are embracing marijuana as more "natural", but this is far from being the case. One recent study found that marijuana induces just as much cell toxicity and DNA damage as cigarette smoke.  The researchers were very clear that marijuana displayed just as much cancer causing power as the cigarette smoke: "In addition, when corrected for total particulate matter yield, little difference was observed in the mutagenic activity of samples smoked under the extreme vs the standard regime for both tobacco and marijuana condensates".
Did you know you can inexpensively do your own testing for most hormones? The industry leader is Discounted Labs..
I went through the "Make Love Not War" era and pot was front and center stage. Of course, the irony is that there's a lot of guys walking around now not able to "make love", not realizing that the lowered testosterone and decreased nitric oxide of long term marijuana use during that time is behind it all.
NOTE: Researchers have recently found that the THC in marijuana actually effects sperm negatively and literally makes them high. The sperm get "wired" and then literally burn out making it hard for them to achieve their ultimate destination. This may yet another reason why marijuana is tied to reports of infertility (along with the decreased testosterone, LH and FSH of course).
NEW FLASH: Marijuana does not just after your gonads - studies have also shown that it attacks the brain. Researchers have also found that heavy marijuana users have 7% and 12% smaller amygdala and hippocampuses, respectively.  They also have a higher incidence of "starter level" psychotic symptoms.
NEW FLASH #2: Researchers may have found one of the reasons that marijuana lowers testosterone: it decreases dopamine over time.  If you've ever noticed that your heavy potsmoking friends lacked a little motivation, now you know
1) J Theor Biol, 1983 Oct 21, 104(4):685-692, "Effects of marijuana on testosterone in male subjects"
2) Pharmacol Biochem Behav, 1986 Jun, 24(6):1749-54, "Acute effects of smoking marijuana on hormones, subjective effects and performance in male human subjects"
3) Intl J of Impotence Research, 2008, 20(6):566-573, "Early endothelial dysfunction as a marker of vasculogenic erectile dysfunction in young habitual cannabis users"
4) Endocrine Effects of Marijuana, Todd T. Brown, MD, and Adrian S. Dobs, MD, MHS
5) Eur J Pharmacol,1974, 26:111-114; Endokrinologie, 1977, 69:299-305
6) Chem. Res. Toxicol., Article Publication Date (Web): July 17, 2009, "The Genotoxicity of Mainstream and Sidestream Marijuana and Tobacco Smoke Condensates", Rebecca M. Maertens, et. al.
7) Neurobiology of Disease, Dec 2009," Chronic exposure to cannabinoids during adolescence but not during adulthood impairs emotional behaviour and monoaminergic neurotransmission"