I heard that if you smoke at most three cigarettes a day you don't get addicted to smoking. A similar idea was introduced in a How I Met Your Mother episode. Is this true?
To disprove this theory, would you need to find one person who had never smoked more than 3 cigarettes a day, but who is addicted? To prove this theory, would you need to find several people who frequently smoke up to 3 cigarettes a day, but who are not addicted?
Speaking for myself, I'm an ex-addict: I've quit several times, and I've discovered (empirically) that I can't have even one or two without becoming addicted again - which disproves the theory - unless, you discount me as an example because I used to have more than 3 and am therefore arguably 'already addicted'.
The abstract from this paper (Children's loss of autonomy over smoking: the global youth tobacco survey) says,
Edit: Also your question title says "prevent" as if smoking less than 3 is the cause of non-addiction: but instead there may be a different, pre-existing cause. For example, maybe some people are inherently more resistant (and others less resistant) to addiction, for all sorts of reasons (mental, physical, social, etc.). The people who are able to smoke less than 3 per day are the very people who were resistant to addiction. Whereas the people who are prone to addiction might start, saying, "Oh I'll just smoke 3, I won't get hooked": and then they don't stop; and then they can't stop even when they want to.
Non-smokers are obviously not addicted to cigarettes.
As non-smokers smoker 0 cigarettes a day, and 0 is less than 3, there's some truth to the assertion.