The major active ingredient in marijuana may reduce resistance to infection, according to laboratory studies. Experiments with mice reveal that exposure to psycoactive component of marijuana (THC, tetrahydrocannabinol), increased the animals' susceptibility to the bacteria that causes Legionnaire's disease. Artificial THC likewise decreased the animals' immunity to infection. The artificial form of THC is sometimes used as an alternative to marijuana.

For 10 years, University of South Florida researchers in Tampa, have been investigating the effects of marijuana on the immune system. Recently, their studies have focused on determining the molecular basis for these effects. The findings offer new insight into how marijuana may depress the immune system, and may have implications for marijuana use by people whose immune systems are already compromised.