Withdrawal is the process by which the body clears out nicotine and toxins associated with smoking. Daily smokers maintain a certain level of nicotine in the bloodstream, and when quitting, the level of nicotine quickly drops to a level at which withdrawal occurs. Some common symptoms include cravings for nicotine, tension, irritability, trouble concentrating, fatigue, insomnia, increased appetite, and headache. Withdrawal varies from person to person, based on individual factors. The first two weeks are likely to be the hardest as your body heals and adjusts to being smokefree. Some of the symptoms are actually signs that your body is healing and returning to a normal, nicotine and toxin-free state. Most ex-smokers feel much better within 90 days. Symptoms can come and go for most of the first year, though usually with lessening intensity and duration as time passes.