Coffee is not just caffeine; it contains monoamine oxidase inhibitors that greatly increase the potency and addictiveness of various other psychoactive substances - including caffeine.
Since these compounds inhibit both MAO-A and MAO-B, they increase the levels of the neurotransmitters serotonin, dopamine, noradrenaline and adrenaline. They do so by binding to the MAO enzymes, making them unable to break down the neurotransmitters.
This means that any drugs that release these neurotransmitters – i.e. pretty much any drugs – are going to become hugely more active! This is surprisingly uncommon knowledge.
Human monoamine oxidase enzyme inhibition by coffee and ß-carbolines norharman and harman isolated from coffee
http://www.sciencedirect.com/science/article/pii/S0024320505007514Norharman and harman in instant coffee and coffee substitutes
http://www.sciencedirect.com/science/article/pii/S0308814609013806Identification and occurrence of the bioactive ß-carbolines norharman and harman in coffee brews
http://www.tandfonline.com/doi/abs/10.1080/02652030210145892Human monoamine oxidase is inhibited by tobacco smoke: beta-carboline alkaloids act as potent and reversible inhibitors.
http://www.ncbi.nlm.nih.gov/pubmed/15582589Monoamine Oxidase Inhibition Dramatically Increases the Motivation to Self-Administer Nicotine in Rats
http://www.jneurosci.org/content/25/38/8593.abstractTransient behavioral sensitization to nicotine becomes long-lasting with monoamine oxidases inhibitors.
http://www.ncbi.nlm.nih.gov/pubmed/14592678Harman in Alcoholic Beverages: Pharmacological and Toxicological Implications
http://www.ncbi.nlm.nih.gov/pubmed/3067615
Harman and norharman in alcoholism: correlations with psychopathology and long-term changes.
http://www.ncbi.nlm.nih.gov/pubmed/8651457
The role of beta-carbolines (harman/norharman) in heroin addicts
http://www.sciencedirect.com/science/article/pii/0767399X96800769