Common street names:  Pot, grass, weed, Mary Jane, ganja.

Cannabis contains at least 400 different chemicals, but its main mind-altering ingredient is THC (delta-9-tetrahydrocannabionol).  The amount of THC in marijuana determines the drug's strength, and THC levels are affected by a great many factors, including plant type, weather, soil, and time of harvest.  Marijuana is far more potent today than it has been in the past due to sophisticated cannabis cultivation resulting in high levels of THC.

THC content in 1974 averaged around 1 percent and by 1994 it had gone up to 4 percent.   Today THC content can be as high as 7.5 to 24 percent.  

Marijuana is most commonly smoked.  It is usually rolled loosely into cigarettes called "joints" or smoked in a water pipe or "bong".  These days marijuana is sometimes laced with other substances which the user may be unaware of, which can include crack cocaine, crystal meth, or fentanyl.  Other forms of administration of marijuana include brewing it in a tea or eating it in edibles such as baked goods.

Common effects of marijuana include euphoria, perceived slowing of time, dry mouth, conjunctival injection (red eyes), increased heart rate, diminished short-term memory, and impaired coordination and balance.   Moderate doses tend to induce a state of well-being or a dreamy state of relaxation, along with a distorted sense of perception.  Stronger doses can cause more intense reactions such as paranoia and hallucinations.

Most of marijuana's effects tend to wear off in a few hours, but the drug stays in the system longer.  THC is fat-soluble and will accumulate in the fatty tissues in the liver, lungs, testes, and other organs.  In casual users THC may show up in urine tests 3 days later, and in regular users it can show up in urine tests up to 4 weeks later.