Methamphetamine is usually found in the underground markets as powder or crystals. Street meth, like many other street drugs, is often cut or is not meth at all. "Glass" or "Crystal" are terms used to describe street meth that appears as clear crystals instead of powder, reducing the chance that the meth is cut with filler or that it is crushed up amphetamine or methylphenidate (Ritalin) tablets. Street meth dosages vary by how pure the material is, by individual tolerance based on past usage, how frequently it is used, and by differences in individual reaction. Dosages given below are based on pure meth for an infrequent user.

Pharmaceutical methamphetamine, prescribed rarely for obesity and ADD/ADHD, is sold in the US in 5 mg tablets (10 mg tablets have been discontinued in the United States) that are intended to be taken orally (some are sustained release outside the US). Dosages of 10-25 mg per day are common for prescription pharmaceutical methamphetamine taken orally, usually spread out into two doses.

A common (although unhealthy) use pattern for methamphetamine is to redose repeatedly for several days in a row. Depending on whether the intention is to stay awake, remain euphoric, or attempt to continue getting 'rushing' effects, doses are repeated every 3-8 hours (to stay awake) or every .5 to 4 hours (to remain "high"). Redosing often involves taking the same dose as the initial dose. As redosing continues beyond 48 hours, the dosages tend to go up.

Tolerance to meth's effects -- where larger doses are required to achieve similar effects -- increases with frequency of use and with experience. A new user's 15 mg insufflated experience may be very euphoric and stimulating, while a 20 mg bump snorted by a regular user may be barely detectable. Chronic, tolerated users' dosages are generally 2-3 times that of those listed below. These heavy users may go through a gram or more per day.

The euphoric feeling associated with strong doses of meth fades quickly and after several doses in a row or using meth on multiple days in a row, much higher doses are necessary to achieve a similar level of feeling happy and upbeat.

MAO inhibitors and CYP450-2D6 inhibitors can increase the effects of methamphetamine and can dangerously inhibit the metabolism of all amphetamines. Some prescription pharmaceuticals are known for producing this effect and new users or users combining herbs, smart drugs, prescriptions, or psychoactive chemicals should be extremely careful of possible interactions.

Please note that there is substantial evidence that toxicity to the brain's dopamine system occurs with high (or repeated) methamphetamine use.