Caffeine and nicotine have fairly different compositions and origins, but as stimulants, they share some important similarities, especially in relation to patterns of usage. As two of the most common and mutually reinforcing psychoactive substances, they are also often both used at the same time.
The most basic connection between the two drugs is simply that they have similar effects. Both are classified as stimulants because they increase focus and physiological arousal. They are also both somewhat atypical as stimulants because they do not consistently produce the same effects and sometimes can have different and unusual consequences in higher doses. These chemicals have effects across the nervous system, unlike substances such as cocaine and amphetamines that target the central nervous system more directly.
Both caffeine and nicotine are addictive, and consistent users of either drug are likely to develop tolerance and dependence. Withdrawal from both drugs can produce a variety of symptoms, especially craving and irritability. Cigarettes are much more addictive than caffeine products, however, both because nicotine is a more addictive chemical and because smoking as a delivery mechanism is inherently more likely to produce addiction.
Various studies, with mixed results, have attempted to determine the connection between these drugs when used together, and they do seem to reinforce each other. The evidence is strong that the consumption of both drugs goes hand-in-hand; people who use one are more likely to use the other. A study on rats confirmed the popular wisdom that caffeine intake increased desire for nicotine. Quitting caffeine along with nicotine amplifies nicotine withdrawal symptoms, but continuing to consume caffeine while attempting to quit smoking is also risky, since the effects of one stimulant can function as a neural cue and generate cravings for the other.
Caffeine and nicotine are widely recognized as mutually reinforcing drugs. Popular culture contains abundant references to the connection between them, from Otis Redding's 1966 song "Cigarettes and Coffee" to Jim Jarmusch's 2003 movie Coffee and Cigarettes. The jury is still out on whether the connection is stronger than the one between nicotine and alcohol.