I waited a good six months after giving up smoking before taking on my 30 day fast. I knew less about fasting back then and was a lot more cautious. I think I probably had as bad or worse toxins in me this time around from my overdosing on junk food for a few years and I leapt into it with only good results. Certainly the detox symptoms were pretty disgusting and hard to bear at times but overall it was certainly safe.