The high you get from marijuana mimics a process your body already has in place to cool down brain circuits that fire stimulating neurotransmitters, the primary "go" signal used by brain synapses, says Eckerd College biology professor Gregory Gerdeman. It does this by triggering your own endocannabinoid system to release chemicals that bind to receptors throughout your body and brain and send the message to chill out.
The best way to calm down is to take deep breaths and remember that the feeling will pass. Other helpful tips include talking to a friend, going to sleep or doing something relaxing like walking. Physical activity can help because it increases blood flow and metabolizes THC faster. However, if you are very uncomfortable or panicked, you may need to seek professional medical attention.