Actually, fentanyl has a very short half-life, which is one reason it's commonly used for twilight sedation for outpatient procedures. It's the method of delivery through the patch that makes it last 72 hours, not the molecule itself. If you've had the patch off for three days and feel fine, you're in great shape!
A month of using Fentanyl won't get you physically hooked. You may experience mild mental cravings, fatiigue, and possibly body aches and rebound pain, but withdrawls should not happen. Really people fear withdrawls after 6 months or even a year of use and often Suboxone and methodone is overly prescribed. Yearss ago it was harder to get on methadone for really your body won't go through medically unstable withdrawls until yeras of using and opiod. Not something every addict knows :) I trully hope the vivtrol injections work for you, I applaud you for at least giving them a shot. Best of luck.
Sara RN