Caffeine enhances the reinforcing effects of cocaine and its motivational value, according to a study published this August in Psychopharmacology. The study on rats points to important implications that these effects may have in humans.

It is well known that illegal drugs are often sold with other substances mixed together with the main psychoactive ingredient, with these sometimes being active ingredients (adulterants). Several forensic studies have reported that caffeine is one of the most common psychoactive adulterants found in cocaine – either in its snorted (hydrochloride) or smoked forms (cocapaste or crack). It is believed to be added to increase the weight and volume but also to mimic or increase the effects of cocaine.

Research also supports this: animal studies have demonstrated that caffeine is able to potentiate several cocaine actions; and in human laboratory studies, caffeine has be found to increase the rate of positive subjective effects in subjects with a previous history of cocaine abuse.

The study, led by José Prieto of the Biological Research Institute Clemente Estable in Uruguay, compared the effects on self-administration of solutions containing cocaine and caffeine, cocaine alone, and caffeine alone. Rats were taught to self-administer a liquid solution by using their nose to poke into a hole; either over an active hole (yellow/ green light) or an inactive hole (red light). The effects of the drugs were then tested in three parts: firstly, when a fixed number of nose pokes were needed to receive the solution; secondly, when the number progressively increased; and finally, as the drugs were gradually substituted for a saline solution.

The results revealed that during the fixed response condition, caffeine did not affect the response to cocaine – there was a similar pattern to the self-administration of cocaine alone. Interestingly however, in the progressive condition, caffeine was able to potentiate the self-administration of cocaine, i.e. rats self-administering the combination of cocaine and caffeine significantly increased their intake of the solution compared to the cocaine only group. Finally, as the drugs were reduced, they found that although caffeine by itself did not maintain self-administration behavior in rats, it maintained drug-seeking behavior of rats previously exposed to combinations of cocaine and caffeine.

The findings suggest that caffeine enhances the reinforcing effects of cocaine and its motivational value. Therefore, it highlights the role of active adulterants commonly used in cocaine-based street drugs and points to important implications that these effects may have in humans.