Naltrexone will cause withdrawal if you have opioids in your blood.
Your doctor may perform a challenge test
before you begin taking naltrexone. A small dose of naloxone
(a drug similar to naltrexone) will be injected into your vein or under your skin, and you will be watched for symptoms of drug withdrawal. If you have symptoms, you will need to wait a few days before beginning naltrexone.
You should take naltrexone as directed by your physician.