Methadone can be highly addictive and dangerous and has been linked to the deaths of several drug addicts' children.
Methadone is used to assist heroin users back to normal lives
So why does the NHS pay for it to be routinely prescribed to thousands of heroin addicts across the country?
WHAT IS METHADONE?
Methadone is one of several synthetic opiates which are known as opioids. They are produced for medical use and have an effect that is similar to heroin.
Opiates and opioids, like heroin and methadone, are sedative drugs that depress the nervous system.
The result is they dull physical and psychological pain, which gives a feeling of warmth, relaxation and detachment. Some users describe the effect as being like wrapped in a warm blanket.
Methadone and its sister drug buprenorphine are prescribed to addicts as a substitute to heroin.
WHY IS IT PRESCRIBED TO HEROIN ADDICTS?
Methadone prescription is official policy for tackling heroin withdrawal. Its supporters say that despite its disadvantages, it gives the best hope of breaking the chaotic cycle of hardcore heroin use.
Methadone has been described as a nicotine patch for heroin addicts - it provides a slow, steady delivery that allows addicts to stabilise their cravings.
Usually taken as a syrup once a day, methadone helps replace frequent daily injections of heroin, and the accompanying crime that addicts commit to pay for it.
In other words, it replaces the sharp highs and desperate cravings that are hard to resist, with a slow, drawn-out rise and fall.
HOW MANY PEOPLE ARE ON IT?
Experts believe there are about 300,000 "problem" heroin users in the country - those whose addictions are damaging the lives of themselves or others around them.
Of these, about half are in some form of treatment, and two thirds of those in treatment use either methadone or buprenorphine.
The cost to the tax payer is thought to be between £3,000 and £4,000 per patient, per year.
DOES IT WORK?
Most experts agree that if an addict wants to kick their heroin addiction, then methadone is an effective way to stabilise their life and their cravings.
But just like nicotine patches, methadone does not mean someone will stop and it does not make them stop. It just gives them an opportunity that would otherwise be very difficult to find.
And although methadone has been shown to reduce chronic heroin use and the frequent crime that accompanies it, the drug is less successful at stopping people from taking drugs altogether.
Because methadone does not give the extreme high of heroin, some people continue to use heroin and only use methadone as a "top up" drug.
WHAT ARE THE DISADVANTAGES OF METHADONE?
Critics say methadone simply replaces one dependency with another, and some say methadone can be even harder to quit than heroin.
The drug is also highly toxic. It can make people sick when they first take it and high doses lead to drowsiness.
It is possible to overdose on methadone, leading to a coma or even to stopping breathing altogether.
Several children have died after ingesting methadone after their addict parents took their prescriptions home with them.
Those who support methadone prescriptions say this could be avoided by making sure addicts are only given their doses under supervision at clinics or pharmacies.
Politicians have also criticised methadone on moral and ideological grounds.
Scottish Conservative Party justice spokesman, Bill Aitken, recently described many of those in methadone programmes in Scotland as sitting "fat, dumb and happy" on the drug.
He said there should be a greater emphasis on abstinence based treatments.