In addition to messing up the brain, it isn’t too great for the lungs

There are 33 cancer-causing chemicals contained in marijuana. Marijuana smoke also deposits tar into the lungs.

http://www.lung.org/associations/states/colorado/tobacco/marijuana.html

“Marijuana is a drug made from the dry, shredded parts of the Cannabis sativa hemp plant.  It is usually smoked in hand-rolled cigarettes called joints, in pipes, or in water pipes called bongs. It is also smoked in blunts, which are hollowed-out cigars filled with a mixture of tobacco and marijuana.

Marijuana contains a potent chemical called delta-9-tetrahydrocannabinol, more commonly known as THC. It’s very similar to chemicals that the brain naturally produces, and disrupts the function of these chemicals in the brain.
Marijuana today is more potent than marijuana of past decades. For a long time THC levels averaged 2.3 percent. Today, average THC levels are higher than 8 percent and can go up to 35 percent in medical marijuana.”