NICE guidelines on opioid detoxification have methadone or buprenorphine as first line treatments, with lofexidine a possible alternative. Methadone and buprenorphine are also considered equally effective as substitution therapy in opioid dependence, although methadone may be more effective in keeping people in treatment.^11,12

High dose methadone or buprenorphine may be more suitable for people wanting to stop heroin completely, while low dose methadone can be used for people who will still be using some heroin. People using high levels of heroin tend not to settle as well with high-dose buprenorphine as they do high-dose methadone, but buprenorphine is less likely to be affected by medicines affecting liver enzyme activity.¹²

Methadone is a synthetic opioid with strong opioid agonist acting at mu receptors, but with some activity at kappa and delta opiate receptors. It will cause a dependence syndrome similar to morphine, but withdrawal symptoms can be much slower than with other opioids.^13,14

Buprenorphine has partial opioid activity at mu receptors and is a kappa and delta receptor antagonist. Its activity in opioid maintenance treatment is thought to be mainly due to its slowly reversible link with mu receptors which reduces opioid need for a longer period in dependent patients.^13,14

Drug half-life (T_½) data indicate that around 90% of a single dose of morphine will be cleared by the body within 24 hours, as its half-life is two hours (but longer for its metabolised forms). Diamorphine’s half-life is only 2-3 minutes but it is metabolised into monoacetylmorphine and then morphine.¹³

By contrast, methadone has a single dose half-life of 12-18 hours and regular dosing will build a methadone ‘reservoir’ in tissues extending the half-life to 1-2 days. Buprenorphine’s plasma half-life is around 37 hours due to reabsorption and it being highly lipophilic.¹³