People who use opiates for pain, (and fentanyl is one very potent opiate/narcotic), often have sweating when their dose is wearing off, or they are becoming tolerant of that dose. When in a real withdrawal from opiates, the sweating can be enormous.

Fentanyl is not indicated for the opiate "naive", meaning the patients should have been on a relatively high dose of narcotic for a while, before considering fentanyl.

Sweating can also come from impaired glucose tolerance. This precedes diabetes, and often happens when blood sugars fall suddenly from a high value to a more normal number, or become low from swinging high to low from a sugar intake. This type of sweating can occur when eating a large meal and is called gustatory sweating, and often only involves the head and neck area.

There are many drugs that can cause sweating. Some people who have inflammation in the body will sweat when they use aspirin or another NSAID in high doses.