Background: Cetirizine, 2nd generation antihistamine, has less central adverse effects compared with the first generation but is not completely devoid of sedative effect. Electroencephalography (EEG) is one of the tests to assess sedation. Aims: The aim of this study was to find and compare the EEG changes in drowsy and non-drowsy subjects after cetirizine administration. Methods and Material: A crossover, placebo-control, double-blind study was conducted on consenting 30 healthy male volunteers. We subjected three (baseline, placebo, cetirizine) 5-min EEG recordings in eye closed condition to fast Fourier transformation and divided EEG frequencies into slow (0.5-6.5 Hz), extended alpha (6.5-14 Hz), alpha1 (6.5-8 Hz), alpha2 (8.5-10 Hz), alpha3 (10.5-12 Hz), alpha4 (12.5-14 Hz) and beta (14.5-32 Hz). Statistical analysis used: The statistical analysis was done using Friedman followed by multiple comparisons. Results: Nine out of thirty subjects developed symptoms of drowsiness after cetirizine administration. In drowsy subjects EEG beta, extended alpha and its subsegment alpha 2 and alpha 3 activities significantly decreased in cetirizine treated condition as compared to baseline. In non-drowsy subjects, there was significant increase in EEG slow and alpha1 activity in cetirizine treated condition as compared to baseline. There was significant decrease in EEG alpha3 activity at most of the sites when EEG activities between nondrowsy and drowsy subjects were compared. Conclusion: Our study suggested that cetirizine most likely decreases EEG power of alpha2, alpha3 and beta activities (i.e. above 8.5 Hz) in subjects experiencing drowsiness and increases EEG slow and alpha1 activities (i.e. below 8.5 Hz) despite no symptoms of drowsiness. On comparison, EEG alpha3 activity decreased in symptomatic as compared to asymptomatic subjects.