MOM - Agonist & Antagonist

• Agonist 


  A drug that activates receptors in the brain, mimics/imitates the action of another chemical or substance
• Antagonist

  A drug that blocks or stops a reaction by binding to receptors and blocking rather than activating them
• Example of Agonist

  Methadone
• Example of Antagonist

  Naltrexone
• What is methadone used to treat?

  Heroin (opioid) addiction
• What is naltrexone used to treat?

  Opioid addiction (heroin), alcohol and has been trialled with gambling.
• • When methadone is taken it binds to opioid receptors and activates them in a similar way to heroin
  • Causes enough stimulation to reduce cravings for heroin and prevents addict going into withdrawal
  • Methadone is stored in liver and bloodstream and is released more slowly than heroin - this means the addict gets less highs and crashes 
  • Methadone occupies opioid receptor sites for longer to discourage use of other opioid drugs
  • Methadone maintains opioid addiction in a safer way until the person can achieve abstinence.
• • Naltrexone binds to opioid receptors and blocks the effects of the AB if it is used
  • This means that if the person uses their AB they will not get any effect from it, including no increase in dopamine
  • This makes the AB less rewarding - so the person is less likely to engage
  • Naltrexone reduces cravings - as the AB no longer leads to pleasure
  • Naltrexone brings on a quicker withdrawal - to get this over with quicker
  • Should be used when the addict is abstinent from their AB (no longer using/engaging)
• Dosage - Methadone

  Administered by medical professional (pharmacist) under supervision

  Orally - green liquid (can be given as pill/tablet)

  Dosage - 10-40mg up to 120mg max (depending on tolerance for heroin)

  Dosage is gradually reduced to achieve detox
• Dosage - Naltrexone

  Oral tablet taken daily

  Depot injection (slower release)

  Implant (though this is not approved in UK)
• • Agonist
  • Methadone
  • Opioid addiction
  • Heroin
  • Mimic
  • Withdrawal symptoms
  • Highs and crashes
  • Green liquid
  • Administered by pharmacist
  • A/A* - NICE 
• • Antagonist
  • Naltrexone
  • Blocks
  • Excitatory neurotransmitters – dopamine
  • Mesolimbic pathway
  • Opioid, alcohol, gambling
  • Orally
  • Implant
  • Less rewarding
  • Reduces cravings highly motivated
  • A/A* - NICE
• Effectiveness

  NICE assessed 31 reviews of effectiveness of methadone compared to a placebo and to no treatment. Methadone = higher retention to treatment and lower illicit opioid use.
• Effectiveness

  NICE reviewed 17 studies assessing the use of naltrexone in heroin addiction compared to a placebo. Results for retention to treatment were mixed in terms of success. But did show some patients had lower relapse in the naltrexone condition if they were highly motivated and well monitored.
• Effectiveness

  Methadone is a maintenance therapy, it is used as a heroin substitute to prevent patients from engaging in illicit drug use. Does this mean it is a cure for addiction? What are the implications of this?
• Effectiveness

  There are methodological issues when looking at research into the effectiveness of agonist and antagonist substitution. Studies tend to be in the short term without long term follow up. Why is this problematic when considering addiction?
• Both pose a risk of harm

  • Side effects - methadone = respiratory & stomach issues, naltrexone = affect liver function
  • Increased risk of OD - 416 deaths in 2018 linked to methadone.
  • But, 1336 deaths in 2018 with heroin OD recorded on death certificate
• Reduces risky AB

  Methadone = safer than heroin (OD, infection, use of needles)

  Naltrexone = stops use of heroin & alcohol
• Only deals with symptoms and not root cause

  Methadone = maintains AB by mimicking heroin

  Naltrexone = blocks pleasure and makes AB less rewarding
• Social - Cheap & Convenient

  If not using illicit opioids, they are more able to function in day to day life & less likely to engage in CB. Reducing costs in relation to dealing with drug related crimes.
• Social - Lack of long-term evidence

  Centre for Policy report - Methadone is an expensive failure, costing the UK up to £1 billion per year not achieving abstinence & addicts remain on the programme for years as well as using other substances.
• Society need to change how it views and approaches drug treatment


  Johan Hari - we need to spend money investing reconnecting addicts with society rather than cutting them off. Portugal decriminalised drug use, reintegrated addicts back into society and had lower levels of heroin use & lower levels of overdose.