I have decided to stop taking Actonel; are there any withdrawal symptoms

Actonel is a bisphosphonate, which as a class is very poorly absorbed and therefore requires that some very strict instructions be followed when taking it to strengthen one's bones, eg treat or prevent osteoporosis.  I am not aware of any withdrawal symptoms or syndrome from stopping this medication abruptly, most likely due to its very long half-life which allows it to be taken anywhere from once a week to once a month to even once a quarter or year (depending upon route of administration).  

However, it's best that you ask your prescribing physician this same question.  It's probably still a good idea for you to get regular DEXA scans (about every 2 years here in the States) and continue taking your calcium.  You should also consider checking your vitamin D level and taking enough to keep your level normalized/optimized.

Hope this helps.  I've written some posts regarding osteoporosis at .