He said methadone did not give its users a high, which is why it was surprising the drug had a street value.
"It actually doesn't do the things that heroin or those other opioid-based drugs do."
Yet there is a market for methadone.
Mr Wood said the Class B drug was easier to source than other narcotics and probably a bit cheaper as well.
Methadone hydrochloride is dispensed through pharmacies to registered users and taken orally as a liquid. The drug is gradually released into the bloodstream: it takes about three days before the maximum effect of an initial dose is felt.
Methadone was first used to treat opioid addiction in the United States in the 1960s. The first New Zealand methadone clinics opened in 1971.
Mr Wood said the methadone programme's purpose was in curbing the illegal activities associated with high-end drug use.
"That's why the programme exists," Mr Wood said. "Because it actually reduces the harm for the community."
Opioid substitution treatment services are funded by the Ministry of Health and predominantly provided through district health boards.
Mr Wood said those presenting to Whanganui DHB's opioid substitution programme had drug dependencies they wanted to do something about.
Some had made contact with the Alcohol & Other Drugs Service, others were referred to it by their GPs or criminal justice agencies.
The first step involved getting the client on a regular dosage of methadone, the size of which varied from person to person and took some trial and error to work out.
Mr Wood said methadone was a potent drug. The first weeks of the programme were the most dangerous.
Once the client had been stabilised, the process of slowly reducing their dosage began.
Ideally, they would be on methadone for no more than five years, because it was addictive and, like all drugs, had side effects.
"We know it's no good for them," Mr Wood said. "It can have a really bad effect on their teeth, for example."
But methadone was a better alternative to illicit drugs, he said.
"There are people on the methadone programme who hold down really good jobs, who are really valuable members of our society, and others that are really struggling."
Mr Wood said putting people on the methadone programme was a last resort.
"They talk about it being handcuffs for life. For some people, they will be on the programme for a long time."
Those who had been taking methadone for several years could find it quite hard to reduce their dosage, Mr Wood said.
"A lot would have tried to get off, but it just becomes physically too painful. They get stuck on it for life."
Clive Kenneth Beach, who was sentenced to 10 months' home detention in the High Court at Wanganui last December for supplying the methadone linked to Sanchia Wilson's fatal overdose, had been on an opioid substitution programme for more than 20 years.
Beach, 58, had started diverting and dealing methadone prescribed for his use just months before Miss Wilson died in April 2012.
Justice Robert Dobson said the offending had the wider implication of casting doubt on drug-replacement programmes.
Beach would collect his prescription on a Monday, Wednesday and Friday, and was entrusted to take the drug on every other day of the week.
Mr Wood said Beach's drug dealing was "not a good look for us".
"We work to national guidelines, but there are many challenges," he said.
"Methadone's a tiny, tiny part of what we do."
There was no point even trying to deal with a drug dependency unless a client's basic needs were being met. "So we work with other agencies on things like housing and food."
Mr Wood said the opioid substitution programme was highly valued by the people who were on it. "They are less likely to get involved in illegal activities. We know that, because it's what clients tell us."
