I was taking Seroquel for sleep and because of major side-effects I insisted that I stop taking it. Pdoc wanted me to continue on it. I stopped it, but I couldn't sleep. I couldn't sleep well for many months, nearly a year. Finally I became desperate; I had some Seroquel left in a bottle. So I took a tiny bit (between 12.5 and 25mg) and, of course, got terrific sleep. Today I requested that my pdoc give me a refill. I hate taking the Seroquel, yet I hate not sleeping even more.
I guess what I'm saying is that it all depends upon why you're no longer on a certain med. For me, it was my own choice.
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I just realized I didn't exactly answer the question you've asked. If I didn't know why the medication was no longer prescribed I would be cautious about taking it prn. Of course, it depends upon other meds you're on, too. Since your NP is out this week, ask your pharmacist for their opinion.