It is commonly believed by these shapeshifters, particularly werewolves, that the full moon bestows them their power that their abilities are at their peak, but at the cost of heightened aggression and bloodlust.
"There's a price you pay for this kind of power. You get the ability to heal, but tonight, you're gonna want to kill anything you can find." - Derek Hale
Born werewolves can learn to control their animal instincts and aggression on the full moon as it's part of their nature. Recently bitten wolves however, have a harder time controlling their transformations and bloodlust during their early full moons, making it a particularly dangerous time to be around people. With time, training, practice and concentration on an anchor, the new shapeshifter can learn to control himself.
Full moons are also a source of power for werewolves. On the full moon in March, the Worm Moon, Peter Hale, in a ritual conducted by Lydia, came back to life.