Both cigarette smoking and chewing smokeless tobacco deliver an immediate dose of nicotine into the bloodstream. Since nicotine is usually both physically and psychologically addictive, attempts to discontinue tobacco product use can be extremely difficult and involve significant withdrawal symptoms. Troubling withdrawal symptoms may begin within hours of the last cigarette or 'chew,' and can include all or some of the following: headache, irritability, depression, difficulty concentrating, increased appetite, and sleeplessness. Sometimes, in order to successfully quit smoking, it's helpful to reduce the severity and duration of these side effects by taking controlled, low doses of nicotine in another form. Various nicotine replacement products, or NRT (N-R-T) products, are available for this purpose in the form of either trans-dermal skin patches or chewing gum. The trans-dermal patches deliver nicotine steadily and automatically through the skin, and may therefore be easier and more effective for some people. For effectiveness, the nicotine gum should be chewed specifically in accordance with package directions. Often, NRT's assist in smoking cessation, since their slow nicotine delivery and resulting low overall blood nicotine levels allow the body to gradually reduce its nicotine dependence. Therefore, the amount of NRT required each day may be diminished over a period of six to eight weeks, providing time for new non-smoking habits to be established. Since neither the gum nor the patches include any of the other harmful chemicals, cancerous substances, or other toxins present in tobacco, it's usually considered safe to use the products for as long as necessary in order to successfully wean the body from nicotine. However, behavior change is usually a key factor in the success of any attempt to quit tobacco product use. Therefore, it's often recommended that NRT products be used only in conjunction with a structured, monitored smoking cessation program.