Monaz: Hello, I'm Monaz Mehta, editor in the Cochrane Editorial and Methods department. Nicotine replacement therapy has been used to help people stop smoking for more than 20 years, and a large and growing body of research has tested it. To help cope with this, some of the related Cochrane Reviews have been divided up and in April 2019 the evidence on different doses, durations and modes of delivery was updated into a new review. Here’s one of the authors, Samantha Chepkin from Cochrane UK to tell us what they found.

Samantha: Nicotine replacement therapy, or NRT, is safe and improves a person's chances of quitting smoking. However, there may be differences in the effectiveness of the many different ways of using NRT. There are different doses and modes of delivery, including skin patches, chewing gum, nasal sprays, mouth sprays, inhalers, lozenges and tablets. There are also questions around how long to take NRT for and whether it is more effective to start on the day of quitting smoking or earlier. We look at all these issues in our systematic review, to see which of ways best for helping people quit smoking for at least six months.
We included 63 randomised trials, involving more than 41,000 smokers who wanted to quit. This provides high-certainty evidence that people who smoke have the same chances of quitting whether they use a nicotine patch or a fast-acting type of NRT, such as gum, lozenge or nasal spray. However, using both a nicotine patch and a fast-acting NRT at the same time made quitting 15% to 36% more likely.  People were also more likely to quit if they used higher‐dose nicotine gum compared with lower-dose gum, although there is some evidence to suggest this may only be true for highly-dependent smokers. 
The evidence is less certain for higher dose nicotine patches, but people are probably more likely to quit with 21/25mg patches than 14/15mg patches.
There is evidence that starting to use NRT before a quit day, known as “preloading”, may help more people to quit than starting on the quit day itself, but more evidence is needed to strengthen this conclusion. 
More research is also needed to answer how long NRT should be used for, whether fast-acting NRT should be used on a fixed schedule or as wanted, and whether more people stop smoking when NRT is provided for free rather than them having to buy it. We found few studies comparing side effects across different types of NRT use; but the side effects that were reported were generally minor. These included skin irritation with patch use.
In summary, our review provides evidence that combination NRT works better than a single form of NRT; higher‐dose works better than lower‐dose; starting NRT before a quit day may result in higher quit rates, and nicotine patch use results in similar quit rates to fast-acting NRT such as gum or lozenge. 

Monaz: If you would like to look at this large collection of evidence on NRT in more detail, just visit Cochrane Library dot com, where a simple search for 'doses of NRT' will show you the review.