Help medicine reactions

I can’t take any meds that can possibly really help me because I have extreme reactions to them. My question is why does the medicine work so well for either one or two days and then the side effects? They all are the same extreme nausea, irritability, low appetite and my light and screen sensitivity comes back. Why after a few days? Someone told me it could be I’m low on some vitamins that the medicine uses? Does that make sense and how can I find out if I’m low my blood tests are all normal? Anybody find a way to not react so badly? I really need these meds to work