The nicotine in cigarettes and other tobacco products makes people dependent. When they quit, they will get withdrawal symptoms. Nicotine replacement therapy (NRT) is there to reduce the dependency symptoms. This is usually done by delivering nicotine (or a substance very similar to it) in other ways. Common methods include nicotine patches and chewing gums with nicotine in them.

Cigarettes are responsible for the deaths of about 5 million people each year. These people are not killed by the nicotine in the cigarette, but by other substances in tobacco smoke such as carbon monoxide and tars. 

NRT delivers nicotine to the smoker's brain in a much slower way than cigarettes do. It helps to damp down the urges to smoke that most smokers have in the early days and weeks after quitting, rather than remove them totally. It gives the smoker the chance to break smoking cues in their daily lives, and might provide a more comfortable exit from the smoking habit. NRT however is best used with some form of support, ideally from someone who knows something about smoking cessation.

In 2005 the Committee on the Safety of Medicines recommended that NRT be given to pregnant smokers and also to adolescent smokers. 

Findings from a recent Cochrane review of controlled trials testing NRT products (Stead et al. 2008) indicated that smokers using NRT were 1.5 to 2 times more likely to be abstinent from smoking at followup than those in the placebo or control treatment condition.

Criticism
Nicotine is the addictive drug in tobacco products, so there is a chance when using NRT products, to become addicted to those products in place of the tobacco habit. Generally it is argued that NRT products are safer then smoking tobacco. NRT products are generally expensive, sometimes even more so then cigarettes, so a minority of people may become addicted to the NRT product and turn back to tobacco products to save money.

Types
 Nicotine patch
 Nicotine gum
 Nicotine inhaler
 Nicotine lozenge
 Nicotine spray
 Nicotine sublingual tablet

Electronic cigarette 
An electronic cigarette is an electronic item used instead of cigarettes. It gives a little nicotine, propylene glycol, and sometimes flavours, when you breathe through it. It was invented in China.

The electronic cigarette cannot be classed as an NRT as no recognized tests or studies have taken place. Furthermore WHO (World Health Organisation) has advised that any manufacturer or seller who is marketing the electronic cigarette as an NRT should halt unproven therapy claims. Currently most sellers who adhere to this guideline are marketing the device as a tobacco alternative.

References

Stead LF, Perera R, BullenC,Mant D, Lancaster T.Nicotine replacement therapy for smoking cessation. Cochrane Database of Systematic Reviews 2008, Issue 1. Art. No.: CD000146. DOI: 10.1002/14651858.CD000146.pub3.
Silagy C, Lancaster T, Stead L, Mant D, Fowler G. Nicotine replacement therapy for smoking cessation. The Cochrane Database of Systematic Reviews 2004, Issue 3. Art. No.: CD000146.pub2. DOI: 10.1002/14651858.CD000146.pub2. abstract|full text
 Real-world efficacy of prescription and over-the-counter nicotine replacement therapy, by Shiffman S, et al., published May 2002 in Addiction, Issue 97(5) at Pages 505–516 Entrez PubMed

Other websites
University of Wisconsin Center for Tobacco Research and Intervention 
Review of NRT from Cochrane 
The Best Electronic Cigarettes  A complete guide on which electronic cigarette to buy

Smoking
Medical procedures
Addiction

es:Parche de nicotina