Crystal meth, meth, speed, crank and tweak are the common street names given to methamphetamine. Crystal meth is a highly addictive stimulant drug that affects the central nervous system (CNS) and is illegal in the United States, according to U.S. Department of Justice. Smoking crystal meth causes several harmful effects on the body and mind.

Crystal meth causes decreased appetite, also known as anorexia, which causes dramatic weight loss in methamphetamine users, reports Behavioral Health of the Palm Beaches. Malnourishment often occurs because users will often not eat or replace food by using additional crystal meth.

Crystal meth is a stimulant that increases the intensity of neurons in the brain, which can increase the awareness and wakefulness part of the brain, according to “Core Concepts in Pharmacology.” One physical effect of using crystal meth is reduced fatigue. This drug makes the user feel restless and anxious, causing frequent movements or sometimes excessive talking. Crystal meth can also cause insomnia.

Skin lesions are another effect from smoking crystal meth. The drug’s effect on the CNS causes hallucinations which lead the methamphetamine user to believe there are bugs crawling underneath or on top of the skin. This is often referred to as “crank bug," according to the U.S. Department of Justice. Crystal meth users may continuously scratch or pick at the skin, attempting to rid the skin of the "bugs," which causes open sores on the skin that can lead to infections.

Crystal meth use decreases the amount of saliva, one of the protecting factors of teeth, in the mouth, says the U.S. Department of Justice. Users also tend to consume high quantities of sugared soft drinks and neglect personal hygiene, causing tooth decay. Many methamphetamine users grind their teeth and clinch their jaws, causing the brittle teeth to break. "Meth mouth” describes the extensive tooth decay and broken or loose teeth of many meth users.

Aggressive behavior, rage and paranoia can occur when using stimulants such as a methamphetamine. The National Institute on Drug Abuse says crystal meth can affect the user’s judgment, often leading to unsafe behavior, such as risky sexual behavior, increasing the risk for developing diseases. Crystal meth users can also become depressed and irritable.