3 The technology
3.1 Naltrexone (Nalorex, Bristol-Myers Squibb Pharmaceuticals Ltd) is an opioid antagonist with a high affinity for opioid receptors. It competitively displaces opioid agonists (for example, diamorphine or methadone), blocking the euphoric and other effects of opioids and thereby minimising the positive rewards associated with their use. The 'Summary of product characteristics' (SPC) states that naltrexone is licensed for use as an adjunctive prophylactic treatment for detoxified formerly opioid-dependent people (who have remained opioid free for at least 7–10 days). There are unlicensed long-lasting formulations of naltrexone in development (depot preparations and implants), but these do not fall within the scope of this appraisal.
3.2 Naltrexone is rapidly absorbed, metabolised by the liver and excreted in the urine with an elimination half-life of 4 hours. Liver function tests are recommended before and during naltrexone treatment to check for liver impairment. The SPC states that 'caution should be observed in administering the drug to patients with impaired hepatic or renal function'.
3.3 Naltrexone is associated with opioid withdrawal symptoms if people are opioid dependent. The SPC recommends challenge testing with naloxone hydrochloride (a shorter-acting injectable opioid antagonist) to screen for the presence of opioids if it is not certain whether the person is detoxified. People may be at risk of a fatal overdose caused by respiratory depression if they relapse while taking naltrexone. This can happen if the person tries a larger dose of diamorphine to achieve euphoria, or if they return to diamorphine use after naltrexone treatment, because of loss of tolerance to diamorphine. For full details of side effects and contraindications, see the SPC.
3.4 The cost of naltrexone is £1.52 per 50-mg tablet excluding VAT ('British national formulary' [BNF], edition 51). People should receive 25 mg naltrexone on day 1 followed by 50 mg daily thereafter for an initial period of 3 months. However, extended treatment may be necessary because time to full recovery from opioid dependence is variable. A three-times-a-week dosing schedule may be considered if it is thought likely to improve compliance with treatment. Costs may vary in different settings because of negotiated procurement discounts.