Crystal meth is an illegal substance that is highly addictive. Although based on methamphetamine, which has some medical uses, crystal meth is often made in amateur labs and can be very risky to use. Someone abusing this drug may seem euphoric, energetic, confused, or aggressive. Someone who has developed an addiction to the drug may begin to struggle at home, work or school, develop a tolerance to the drug and spend a lot of time trying to get more. Addiction can also lead to overdose, which may cause a fever, shaking, an irregular heartbeat, seizures, or coma.