Smoking marijuana regularly (a joint a day) can damage the cells in the bronchial passages which protect the body against inhaled microorganisms and decrease the ability of the immune cells in the lungs to fight off fungi, bacteria, and tumor cells. Marijuana use also impairs perception, judgment, thinking, memory, and learning. (10)
Alcohol use also affects the immune system as a whole, and also impairs perception, judgment thinking and learning. In addition, while the average joint has 0 calories, the average beer has about 150 calories, which can contribute to weight gain, not to mention hangovers.