I wish I knew more about the med you are asking about so I could be of more help.
I just wanted to share that the Fentanyl can certainly make a person very drowsy. I was tired all the time also when I was on the higher dose of 100mcg.
Now I'm down to 50's and don't seem to get drowsy as much.

If you suspect there might be something else going on, it might be wise to get your thyroid checked. So many things went wrong with me when it was discovered that I was hypothyroid. The main thing it caused was chronic drowsiness, no energy, hair falling out and weight gain.

Now that I'm on the proper med for my thyroid function, I feel much better.....except for the pain issues - grrrrr

Just a thought
Caring,
Rae