I'm going to have back surgery next week. Is there any restrictions on the type of pain medication that they will try to give to me?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Using an anesthetic that contains alcohol before operating a surgical operation is permissible if there is no alternate, but such a patient should use it only to the extent of his need.
Imam al-Nawawi in his book "al-Majmooa" wrote, 'al-Shafi'iyah said drinking medicine that causes loss of mind is permissible at the time of necessity, in this case, such a person has not to make up the missed prayers at the time of losing his mind. Because he lost his mind for a lawful reason. If one whose hand is infected with cancroids needs to use some anesthetic to have his hand cut off, he may use it according to the most correct opinion'.
Imam Ibn Rajab al-Hanbali mentioned in his book "Jame al-Uloom wa al-Hikam" using an intoxicant that causes loss of mind and does not give any pleasure such as henbane is permissible for the purpose of medication if its usefulness is proved by experience.
Allah knows best.