Would I be able to use an inhaler on Shabbat? Would I be able to use a nebulizer on Shabbat and if so would I be allowed to turn it off after being used? Who can administer the medication?

ב"ה Shalom and Refu'a Shelima Just so there are no misunderstandings, my assumption is that the inhaler you are referring to is simply the medicine, which is breathed in by the patient when needed, not an electrical device. This medicine may be taken on Shabbat .(Shemirat Shabbat kehilchata Chapter 33:23, New edition) I assume your nebulizer is electric, so you should connect to a Shabbat timer, which can put on and shut it off by the hours which were preset before Shabbat. In case of emergency it may be put on on Shabbat as well. All the best and Refu'a Shleima