The only withdrawal symptom could be worsening of your asthma. Smoking cigarettes poses serious risks for everyone, but especially for anyone who has a chronic respiratory disease, such as your asthma. The respiratory risk is both immediate -- poor control of asthma and long-term -- the progressive loss of normal lung function. Seriously consider smoking cessation.
You may want to look at our Quit Smoking Topic Center at http://www.nationaljewish.org/topic/smoking_cessation.html for ways to help you quit smoking. Also check with your doctor for other quit smoking resources in your area.
The side effects from the Singulair were too much for me to handle. I guess I'll keep searching for other alternatives.