Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your health concerns.. As per your complain Oxycodone which is a Narcotic Pain killer used to treat moderate to severe pain and Gabapentin which is an anticonvulsant and painkiller both has a common side effect of causing reduced saliva secretion leading to DRY MOUTH.. Burning mouth and Thrush both occurs commonly in patients with dry mouth and when you take both the medicines together there is exaggeration of the side effects of dry mouth leading to outbreak of fungal infection and burning of mouth.. -I would suggest you to consult your physician and get evaluated so that he can either change one or both of the medicines or reduce their dose.. -You should avoid taking both the medicines together.. -For thrush oral antifungal medicines like Fluconazole and Itraconazole has to be taken, Nystatin mouthwash will help, taking more probiotics like yoghurt and garlic will help in clearing the thrush.. -For burning mouth sucking ice pops and drinking cold water will provide relief.. -Drink plenty of water to maintain hydration.. Hope this information helps and thanks for writing to Healthcare Magic.. If you find the answer the please give a Five Star Review and click on found the answer helpful as a token of appreciation.. Thanks and regards. Dr.Honey Nandwani Arora..