Opioid substitutes

 

Opioid substitution aims to stabilise the users' current drug intake and lifestyle and to promote change, while preventing illegal drug use and reducing associated “risky” behaviour.

 

Substitute medications are prescribed when the patient shows signs of withdrawal after abstinence from the opiate, which can indicate dependence. They also admit to regular use of opioids, along with a physical examination and toxicology report confirming this.

 

The substitute can reduce or prevent withdrawal symptoms and maintains an established link with the patient and supportive services.The prescriber must be satisfied that the addict is motivated to change, is capable of complying with the regime and is not already receiving opiate substitutes from another source.

 

Methadone or buprenorphine are usually used as opioid substitutes.

 

Methadone

Methadone is a synthetic, long acting, μ opioid agonist. It bears no chemical relationship to morphine, but they both exhibit similar structural conformation. It has a longer half-life than heroin; therefore it can reduce the symptoms of withdrawal.

 

Despite methadone being a pure opioid agonist, it also exhibits activity at non- opioid receptors, resulting in a wide range of side effects such as respiratory depression, euphoria, sedation and dependence.

 

The initial dose is 10-40mg daily, which can be increased by up to 10mg daily (to a total maximum weekly increase of 30mg) until no signs of withdrawal or intoxication seen, with the  standard maintenance range of 60-120mg daily.

 

Buprenorphine

Buprenorphine is a thebaine derivative which structurally resembles morphine. However, it is a partial agonist which binds almost irreversibly to opioid receptors, therefore it can act to facilitate withdrawal if heroin is injected concurrently.

 

Due to being a partial agonist, it is less sedating than methadone and induces fewer incidences of respiratory depression.

 

Buprenorphine can be formulated with naloxone, an opioid antagonist, as Suboxone® in order to discourage IV administration. The naloxone component precipitates withdrawal symptoms if the preparation is crushed and injected, but has little effect if taken sublingually; therefore it has less abuse potential than buprenorphine alone.

 

The dose of buprenorphine alone is initially 0.8- 4mg on Day 1 and adjusted by 2-4mg daily to a maintenance dose of between 12-24mg daily. Maximum dose should be 32mg daily.

 

The dose of Suboxone® is 2-4mg daily initially and increased by 2-8mg daily until a maximum of 24mg a day is given. Total weekly doses can be divided.