Which of the following statements is FALSE about Naltrexone?

Ref: KDT 6/e p467 *Naltrexone is chemically related to Naloxone and is a pure opioid antagonist. It is more potent than Naloxone. *It differs from Naloxone in being orally active and having a long duration of action (1-2 days) which is used to prevent relapse of heapy drinking. *Side effects are nausea and headache; high doses can cause hepatotoxicity.