Nicotine receptors take over a month to normalize after quitting
In the study, 19 heavy smokers and 20 non-smokers underwent brain scans using single photon emission topography (SPECT) which can measure the density of beta-2 nicotine receptors. The smokers were also scanned at various time-points after quitting smoking. During the first 4 weeks after quitting, the ex-smokers had 20-30% more nicotine receptors, but the number had normalized to that of never-smokers by weeks 6-12. The time-course of these changes is similar (though not identical) to that consistently found for studies of nicotine withdrawal symptom severity, and may reflect a readjustment process in the brain.
Studies like this one are technically difficult and expensive to do, as brain-scanning itself is an expensive business, and some of the methods for assessing specific nicotine receptor numbers have only recently been developed. But the evidence from this and similar studies supports the idea that nicotine withdrawal is related to the number of vacant nicotine receptors, and that it takes just over a month after quitting for these to normalize.
Of course, this doesn’t mean that nicotine addiction is all over six weeks after the last cigarette. It just means that the acute nicotine withdrawal phase is largely gone within that time frame.
A nice summary of the study with a picture of the brain scans can be found at:
Cosgrove KP, Batis J, Bois F, Maciejewski PK, Esterlis I, Kloczynski T, Stiklus S, Krishnan-Sarin S, O'Malley S, Perry E, Tamagnan G, Seibyl JP, Staley JK. Beta-2 nicotinic acetylcholine receptor availability during cute and prolonged abstinence from tobacco smoking. Arch Gen Psychiatry. 2009 Jun;66(6):666-76