My husband went through a rapid detox, and at the end he had an injection of Naltrexone (aka vivitrol) and then he had another injection the following month. He had no problems with it. Slight nausea, little tired maybe for a few days but that was all. He felt it reduced cravings, and many people say that just knowing you cannot get high (from opiates) while on this; makes it easier to stop thinking about it.
My husband only took it those two times; he was in non 12 step rehab after detox, and had the second injection while there. Then he decided to stop then because he was getting lots of counseling and thought it would be enough for him.
He is 8+ months clean.
Good luck to you both.