Effects of ingesting modafinil, caffeine, or their combination on cognitive and physical performance

Modafinil (M) is a psychostimulant prescription drug with off-label applications for sustaining the alertness of healthy individuals. The purpose of this study was to determine if the combined ingestion of M and caffeine (C) would enhance physical and/or cognitive performance more than ingesting M alone. Fourteen healthy male subjects cycled to exhaustion at ∼85% V˙O2 peak and also underwent two cognitive tests (CT): i) a stop signal task (SST) and ii) an irrelevant sound effect task (ISE). They did this twice per visit (AM and PM) after ingesting either placebo (P), M (200 mg), C (200 mg), or M+C (200 mg each) in a double blind fashion. In the AM, cycle ergometer time to exhaustion (TE) was prolonged by M (33%), C (24%), and M+C (36%), with no differences between M, C, and M+C. In the PM, TE was similar across all treatments. During the SSTs, M (5 %), C (6 %), and M+C (5 %) all accelerated reaction times (RT) to targets but only M+C (25%) reduced response variability. In the AM, M (20 %), C (15 %) and M+C (14%) similarly accelerated stop signal reaction times (SSRT). In the PM, only M+C (10 %) accelerated SSRT. For the ISEs, no differences were present between treatments for serial recall accuracy during either a sound absent or sound present condition across all subjects. However, when subjects were split into low (n=7) and high (n=7) body mass groups, treatments effects were observed during the sound present condition suggesting that relative drug doses are important for working memory and attention. In conclusion, M, C and M+C all enhanced measures of physical and cognitive performance. The ingestion of M+C did not provide a further benefit over M or C alone. The combination of M and C was well tolerated with no obvious or reported side-effects.