I've Been Taking Alprazolam, .50mg For Over A Year. I

I've been taking alprazolam, .50mg for over a year. I have to be prescribed it every month. My last refill went from .50 mg to .25mg. Will i have any side affects? I was taken off of it completely 2years ago, had a really bad episode...thought I was having a heart attack, severe dizziness and unbalanced, even went to the hospital, nothing wrong. Was out back on ,.50mg...within 2 days i was back to normal. I am a female, 60 years of age