Meth is a stimulant and it stimulates the brain so that you will get a high, you get talkative and hyper. When an addict has a meth binge they can stay up all night for days so their brain can get affected by the drug itself and the sleep deprivation. Their brain waves start to go to overactive and it can cause cognitive and sensory problems including pseudo-schitzophrenic delusions and hallucinations and other types of behaviour. One well known hallucination is that their brain starts to make the body itch and so much so they think they are infested with bugs.
