My Husband Has CRPS And Was Started On Fentanyl 25

my husband has CRPS and was started on Fentanyl 25 mg ,He has wore them for about 3 weeks now and still has the dry mouth,sleepiness and just out of it. How long does it take for the side effects to go away or do we need to contact the Dr and lower the dose or just come off all together. He is loving it as this is the first time he has been able to get some pain relief in 15 years