Due to dreadful bladder spasms (I have a complex medical history, bowel problems too, so no oral pain killers really got through to help.)
I was started initially on Fentanyl patches, 5mcg, much too strong for me. I then was tried on BuTrans, 5mcg to start. Great the pains went away. They did take a while for me to feel less of the side effects.
But I got a dreadful headache just above my eyebrows, and I was so very sick. I noticed when I drank milk, or ate anything with milk I instantly got the violent headache and sickness - actual vomiting.
Trial and error I found it was down to the lactose. I bought lactose free milk and avoided all foods etc containing milk. (amazing how many foods etc do have milk in when least expected).
Since using the 5mcg and then eventually going up to 10mcg patches, I have never had a headache or vomiting since. I suggest anyone with similar side effects try a lactose free diet to see how you get on.