It sounds so childish doesn't it, "oh no I'm having nightmares wahhh" lol. But when it's so very real and terrible because of the medication you're on it's a different story. Last night I dreamed all my teeth fell out, I remember I was eating a meal and a tooth came loose, then another, then all of them. In the dream I looked in the mirror, I woke up having a full panic attack as my body was full of adrenaline.
Calmed down pretty fast once I was awake but it gives you some idea of how bad an opiate nightmare can be. There are others I could mention even worse but I won't.