God made pot. Man made beer. Who do you trust?

Why weed is the ♥♥♥♥!
Heart Problems
It is accepted in medical circles today that marijuana use causes no evident long-term cardiovascular problems for normal persons. Marijuana smoking, however, does cause changes in the heart and body's circulation characteristic of stress, which may complicate preexisting cardiovascular problems like hypertension, cerebrovascular disease, and coronary atherosclerosis. Marijuana's effects on blood pressure are complex and inconsistent as of yet.

Hormones
Chronic marijuana use has not been found to alter testosterone or other sex hormone levels. In contrast, heavy alcohol use is known to lower these same testosterone levels.

Reproductive Damage
No trustworthy study has ever shown that marijuana use damages the reproductive system, or causes chromosome breakage. Studies of actual human populations have failed to demonstrate that marijuana adversely affects the reproductive system. Claims that marijuana use may impair hormone production, menstrual cycles, or fertility in females are both unproven and unfounded.

The Immune System
Studies in which lab rats were injected with extremely large quantities of THC (the active compound in marijuana) have found that marijuana (in such unrealistically huge quantities) does have an "immunosuppressive effect" in those lab rats, in that it temporarily shuts off certain cells in the liver called lymphocytes and macrophages. These macrophages are useful in fighting off bacterial, not viral, infections.

But this is only for the duration of intoxication. There also exists some evidence that marijuana metabolites remain in the lungs for up to seven months after smoking has ceased, possibly affecting the immune system of the lungs (but not by turning the cells off).

This said, doctors and researchers are still not sure that the immune system is actually negatively affected in realistic situations since there are no numbers to support the idea. In fact, three studies showed that THC might have actually stimulated the immune system in the people studied.

Cancer
Smoking marijuana has the potential to cause both bronchitis and cancer of the lungs, throat, and neck, but this is generally no different than inhaling any other burnt carbon-containing matter since they all increase the number of lesions (and therefore possible infections) in your airways.

The Gateway Effect
Marijuana use has not been found to act as a gateway drug to the use of harder drugs. Studies show that when the Dutch partially legalized marijuana in the 70's, heroin and cocaine use substantially declined, despite a slight increase in marijuana use.