In America, methamphetamine is clinically recognised as the most destructive drug ever. The take-up rate for heroin and crack cocaine after twice using is 20%; the take-up rate for meth after twice using is as high as 95%.

The, comedown, or "crash", is the "meth" high's flip side; a psychologically and physiologically debilitating experience that occurs when the user's neuronal supply of dopamine is depleted and the brain's natural production shuts down. Effectively cut off from his internal energy source and unable to tap into the right hemisphere of his brain, the user's creativity dries up and he cannot experience or express positive feelings like love, happiness, joy or pleasure. Instead he is plagued by profoundly negativite emotions like paranoia, anxiety, rage and self-doubt. Because crystal considerably amplifies existing negative character traits, the more disillusioned, discontented and depressed with life a person is the greater the impact the crash – and its fallout on his environment – will be.

The crystal meth user may have to wait up to a week or more for his body to manufacture a fresh store of dopamine. During this period his mind will be consumed with negative thoughts and emotions which feed on his fears, irrationalities and insecurities, turning him inward as he projects his anger and hostility at his environment. Effectively, he is at war with the world.

Feeling drained and powerless, unresolved painful issues from the past resurface to torment him, intensifying his escalating psychological turmoil. The only escape from the devastating, long-term effects of the crash is the short-term release promised by the next intake of meth, which the user's mind craves. Persistent crystal ingestion progressively weakens the brain's production of dopamine, and interest in the normal rewards of life fade away as people, places and activities associated with meth using take centre stage.

"The very things that are horrifying about crystal meth to a normal person are alluring to a self-destructive addict. That it is made from outrageously toxic substances added to its outlaw appeal. Staying up for three or four days seemed like a door to a magical universe. And the compulsive behaviour turned normal life into something unimaginably boring. I saw crystal as the ultimate act of rebellion instead of the mundane dead end that it is."
~ Patrick Moore [Former addict and author of Tweaked: A Crystal Meth Memoir]

Soon, the abuser's supply of dopamine is so diminished and he will have developed such a tolerance to crystal that he uses the drug constantly just to feel a semblance of normality and to allay the horrendous effects of the crash. By this stage, he will be using crystal on waking – if his speeding mind has allowed him any sleep the night before – and periodically throughout the day, because his body needs the drug in his system just to feel normal. What started as a pursuit for pleasure has become a near-constant form of self-medication in order to simply function, and to deal with the mountain of otherwise neglected chores such as paying bills and buying groceries.

The caustic acids and acidic gases emitted by meth's ingredients can burn right through to the bone from the user's point of entry.

The abuser's self-perception is markedly distorted by meth. He may be unaware of his physical deterioration and become so intoxicated that he frequently forgets to drink liquids, inducing severe dehydration. Ulcers, dry skin, sores, sweating, dilated pupils, tooth grinding,
vomiting, diarrhoea, compromised blood sugar levels and convulsions manifest, while suppressed appetite and chronic lack of sleep results in malnourishment, severe vitamin depletion and wasting muscle tissue on an increasingly emaciated frame, which the abuser may deludingly register as desirable muscle definition.

Mercilessly ravaged facial features are typified by blotchy, pallid skin and dark rings encircling sunken, hollow eyes; not unlike lipoatrophy, the physical condition caused by some HIV medications. Just a few months abusing meth can physically age someone by ten years or more as they deteriorate into a zombie-like shell of their former self, akin to an internal light being snuffed out. [See Faces of meth]

Crystal meth addiction reaps a dehumanising toll. Meth smokers develop receding gums and rotting teeth that turn grey-brown, dissolve and fall out due to calcium depletion, which can also trigger intense pain arising from the brittle bone disease osteoporosis, usually seen in people over 60. Lead poisoning can also occur in heavy users. Crystal can also cause unsightly abscesses to form due to a build up of poison in the body.

The abuser has difficulty looking others in the eye and at his glazed reflection. He no longer seems to be the same person, appearing wired, or "tweaked" (depressed, irritable, fearful, anxious, compulsive, agitated, unpredictable, nervous), and exhibiting symptoms of severe mental illnesses where previously none existed, such as psychosis and paranoid schizophrenia (panic, violent outbursts and repetitive behaviour patterns).

Profound effects on the central nervous system induce sleep deprivation, confusion, delusions, disturbing auditory and visual hallucinations and suicidal thoughts.
Adverse effects on the cardiovascular system include hyperthermia, hypertension, tachycardia (increased heart reate) and dysrhythmias (uncoordinated heart beat). Overdosing on crystal meth can induce severe convulsions followed by circulatory and respiratory collapse, coma and death. Some peope have died taking just minute doses, and extreme self-neglect can take an irreversible and even fatal toll on the body.

Eventually, crystal meth will completely exhaust the brain's ability to manufacture dopamine, and the abuser will feel permanently dissatisfied with life and its rewards.