The pleasure of methamphetamine is so powerful that the risk of prison does not deter users from feeding their addiction, a world expert on how drugs affect the brain told the Court of Appeal.
And this lack of control damages the frontal lobes of the brain - making people more likely to be violent and commit impulsive crimes such as shoplifting.
That's the evidence of Professor David Nutt, a psychiatrist and professor of neuropsychopharmacology in London who has published more than 500 research papers and books on the effect of drugs on the brain.
His research has changed drug policies in the United States, Finland and Sweden and he has been appointed as an advisor to the United Kingdom government on drug laws.
Professor Nutt was asked by the Criminal Bar Association of New Zealand to provide an expert report on sentencing for methamphetamine offences.
His report was given to the Court of Appeal which is considering a major shake-up of prison sentences for meth offending, in which Class-A drug convictions carry a maximum of life imprisonment.
The effectiveness of long prison sentences in deterring people from methamphetamine offending was questioned in front of the five senior judges, especially for those considered "disposable" by organised crime syndicates or caught in addiction.
Methamphetamine is a stimulant which releases dopamine and noradrenaline to the brain which Professor Nutt said gives users a "high" and explains the alertness, insomnia and, in some cases, paranoia.
Professor Nutt said methamphetamine was once a treatment for asthma. It is rarely used today, although it is sometimes given to patients suffering narcolepsy (a neurological disorder of sudden onset of sleep during the day).
Patients with this disorder have often been on methamphetamine for decades with little evidence of harm, said Professor Nutt.
Soldiers in the armies of the Axis powers in WWII were also given methamphetamine to keep them fighting for days on end without sleep.
This was when the characteristics of heavy use became apparent, said Professor Nutt, especially hallucinations of bugs under skin and paranoia.
"When taken orally as a medicine it builds up quite slowly in the body and rarely gives much of a high," Professor Nutt wrote.
"However, when smoked as the free-base [version] or injected, then the impact is very fast and very strong. It is this that leads to dependence and addiction."
According to analysis of drug "harm to users" in the UK, Professor Nutt said heroin and crack cocaine was more harmful and addictive than methamphetamine.
However, Professor Nutt acknowledged the wider analysis of "harm to others" from each drug - such as crime and economic effects - could not be compared.
This was because methamphetamine was consumed more in New Zealand than the UK.
Professor Nutt also explained methamphetamine addiction was a brain disorder that is hard to overcome once established.
"It does not simply go away on its own by simply stopping someone ... the desire to use is often present for years after stopping because the memories of the effects of methamphetamine, especially when smoked or injected, are so powerfully pleasurable that they never go away," wrote Professor Nutt.
"The desire to use again, even when the person knows that to do so will lead them back into addiction, or even to prison, can be profound and in many cases will overwhelm their intention not to use."
Professor Nutt said a major factor in the cycle of methamphetamine use and addiction is the risk of entering the justice system.
"Even when a person with methamphetamine addiction accepts that they have problems, many are dissuaded from seeking help because of the threat of being arrested and getting criminal punishments.
"This avoidance of seeking treatment therefore increases the use of methamphetamine."
Neglect or abuse in childhood, or trauma, or poverty were also factors in the likelihood of someone taking drugs, said Professor Nutt.
Poorer parts of New Zealand have been hit hard by methamphetamine, according to wastewater data. Image / NZ Police
"The impact of a powerful drug like methamphetamine can be overwhelming to individuals with a fragile and underdeveloped degree of self-control.
"It can take them to a place of pleasure and delight such as they have never experienced before. They immediately know that it is very unlikely they could ever replicate this through normal life."
Methamphetamine overwhelms the brain so normal experiences that are rewarding, such as the love of a partner, or child, become devalued.
"In the extreme cases of addiction the only thing that gives them pleasure is more methamphetamine," said Professor Nutt.
Making this worse, extensive use of methamphetamine diminishes the frontal lobes - the part of the brain which regulates behaviour - which makes users more likely to be violent or commit impulsive crimes.
As such, Professor Nutt said prison is not an effective deterrent for methamphetamine users.
"Once someone is addicted to methamphetamine they have a mental illness and must be treated for this. Prison is not a sensible, effective or humane solution."
He pointed to the success of Portugal in tackling the country's heroin addiction.
More than 15 years ago, Portugal abolished criminal sanctions for those caught with possession of illicit drugs for personal use.
Personal possession offences are now dealt with by civil sanctions, such as a fine or travel ban, unless the person is addicted or has mental health issues, in which case they are offered treatment.
"This approach has been spectacularly successful. The numbers of people in prison for drug possession has fallen significantly, saving the government a great deal of money that can be put back into treatment services," said Professor Nutt.
"Further, the rates of drug use and drug crime have also fallen very significantly."
A rare "full bench" of five Court of Appeal judges, headed by president Justice Kos, has yet to release its decision.
The potentially landmark case comes as police released preliminary data from wastewater testing which shows New Zealanders consume $1.4m of methamphetamine every day.
The possible revision of sentencing for methamphetamine comes as the Government's proposed changes to the Misuse of Drugs Act have been described as de facto decriminalisation for personal use of drugs.
In a move described by the Police Association as an under-the-radar change, the amendment bill would also codify police discretion into law.
It would mean a prosecution for drug use or possession - regardless of which drug - should be pursued only if it was in the public interest, taking into account whether a "health-centred or therapeutic approach would be more beneficial".
