Table 2. The percentage of participants who reported side effects 20 min and 24 h after exercise (n = 16). Placebo Caffeine P value 20 min 24 h 20 min 24 h 20 min 24 h Muscle soreness – 13 – 6 – 1.000 Increased urine output 25 6 31 13 1.000 1.000 Tachycardia and heart palpitations 25 0 19 0 1.000 1.000 Anxiety or nervousness 13 0 13 0 1.000 1.000 Headache 19 6 19 0 1.000 1.000 Gastrointestinal problems 6 0 0 6 1.000 1.000 Insomnia – 6 – 0 – 1.000 Increased vigor/activeness 13 6 38 6 0.125 1.000 Perception of performance improvement 25 – 25 – 1.000 – P values show the results of comparisons between conditions using McNemar’s test for each item at each time point.