Chemicals found in cigarettes are more harmful to your body during periods of fasting because your body starts to turn to these chemicals for nutrients. Compiled of nicotine, carbon monoxide and radioactive nuclides, cigarette smoking increases the risk of nausea, dizziness and vomiting as these components quickly damage your central nervous system. After a week of fasting and smoking, your body is more susceptible to the effects of carbon monoxide as your body is now using stored fat and nutrients, increasing your risk of heart attack or stroke according to Myfasting.com.