The speed of nicotine absorption
Cigarettes are addictive because they contain nicotine. However, when the same substance is used in nicotine replacements, it enables thousands of people every year to kick their addiction. This paradox can be explained by the varying speed at which nicotine may enter the bloodstream and reach the brain.
When a substance is smoked or inhaled, it always reaches the brain via the bloodstream faster than if it had been injected. The substance arrives in the lungs and passes directly into the bloodstream through the alveoli. This blood then travels to the heart just once before being pumped to the organs, including the brain. It therefore takes just 8 to 10 seconds for the nicotine to reach the brain after the smoke is inhaled.
On the other hand, an injected substance always enters the bloodstream via a vein. From there, the blood flows into the heart a first time before being pumped into the lungs, where it picks up Oxygen. It then goes back to the heart before being pumped to the rest of the body. This process takes between 30 and 60 seconds. Furthermore, and more importantly, the arrival of the nicotine in the brain is staggered over time.
For smokers, the concentration of nicotine in the blood continues to increase after the first 'hit' until the whole cigarette has been smoked (which takes about 5 to 7 minutes). After this, the concentration rapidly decreases - after an hour, it will have been reduced by half, and after two hours it will be just a quarter of that reached during the first puffs of the cigarette.
Nicotine replacements provide a weaker concentration of nicotine at a slower speed than a cigarette. Smokeless tobacco or 'snuff', popular in countries such as Sweden, is between the two.
Gums and lozenges act faster than patches because the substance released inside the mouth is absorbed into the saliva and passes quickly into the blood stream. It reaches the brain in about 2 to 3 minutes. For this reason, nicotine gum can be useful when a smoker experiences strong, sudden cravings for nicotine. It is also why there is a (relatively low) risk of becoming dependent on nicotine gum. Research has shown that less than 1% of those who use nicotine gum are dependent on it one year after quitting smoking.
As for nicotine patches, they offer the advantage of releasing nicotine slowly. An hour can go by before the maximum dosage - generally equivalent to one third of the 'high' of a cigarette - is absorbed into the bloodstream. Because of this slow release, it is practically impossible to become dependent on patches.
Effect of smokeless tobacco (snus) on smoking and public health in Sweden. J Foulds, L Ramstrom, M Burke, K Fagerström. Tobacco Control 12, S. 349-359 (2003).