Methamphetamine is a powerful stimulant that activates certain systems in the brain. It is closely related chemically to amphetamine, but the central nervous system effects of methamphetamine are greater. Both drugs have some medical uses, primarily in the treatment of obesity, but their therapeutic use is limited. The central nervous system actions that result from taking even small amounts of methamphetamine include increased wakefulness, increased physical activity, decreased appetite, increased respiration, hypothermia, and euphoria. Other CNS effects include irritability, insomnia, confusion, tremors, convulsions, anxiety, paranoia, and aggressiveness. Hyperthermia and convulsions can result in death.

Methamphetamine can be swallowed, snorted, smoked, dissolved in water and injected. As with all addictive drugs, the potential for addiction is greater when it is delivered by methods that cause the concentration in the blood to rise quickly, principally because the effects desired by the user are felt more quickly and with a higher intensity than a moderated delivery mechanism.

Methamphetamine is a powerful nasal decongestant, so methamphetamine users who snort it often have very clear nasal cavities. However, there have been rare cases of people snorting so much meth that their nose cartilage deteriorates, though snorting cocaine is far more likely to cause nasal degeneration, due to its vasoconstrictive properties. Snorting methamphetamine may also cause tooth decay, since the nasal passages are directly connected to the mouth region, and it is theorized that damaging crystalline particles can still attach to the teeth. Another theory is that the drug directly affects calcium balance in the body.

Methamphetamine is commonly smoked in glass pipes, or in aluminum foil heated by a flame underneath. This method is also known as "chasing the dragon". Methamphetamine must be heated (not burned) to cause the desired smoke. Smoking methamphetamine is probably the most impure form of ingestion. In addition to the possible effects on teeth, it is very damaging to the lungs. Methamphetamine users who smoke it sometimes experience mild asthma, which can be countered by inhaling salbutamol aerosol spray, or epinephrine aerosol. Another problem with smoking meth is the potential presence of oxidation byproducts created when the heated drug comes in contact with air. Even if the initial drug is pure methamphetamine, the act of smoking it produces other chemicals, some of which may be toxic.

Injection is a popular method for use, but potentially carries with it quite serious risks. The hydrochloride salt of methamphetamine is soluble in water; injection users may use any dose from 200mg to over a gram in one I.V. dose using a small needle. In methamphetamine research, injection users often do not experience severe tooth decay, presumably because there is no residue left as with smoking it. But injection users experience greater jaw-clenching than users who snort or smoke it, since injecting methamphetamine has a much more powerful effect. This can cause loose teeth, so injection users still do lose their teeth. Also, this method of ingestion brings the risk of infection; injection users often experience skin rashes (sometimes called "speed bumps") and all kinds of infections due to the methamphetamine damage to the skin. As with any injected drug, if a group of users shares a common needle without sterilization procedures, very grave blood-borne diseases such as HIV or hepatitis can be transmitted as well.

YABA: Thai for "crazy medicine," Yaba, a methamphetamine tablet, has been appearing in the UK. Yaba tablets are sometimes flavoured (grape, orange, and vanilla): tasting like candy, the tablets are obviously marketed to a young audience, particularly at rave’s or parties where Ecstasy (a similar looking drug) has been well established. The tablets are commonly reddish-orange or green, and fit inside the end of a drinking straw. They have a variety of logos, with “WY” the most common. Methamphetamine pills are normally ingested orally, although they can be crushed into powder and administered.