Smoking has been recognized as a major risk factor for respiratory diseases especially, chronic obstructive airway disease, lung cancer and cardiovascular diseases. This study was undertaken to investigate the effects of smoking (cigarette and marijuana) on the peak expiratory flow rate among undergraduates. The study was carried out at both campuses (Igbariam and Uli) of Chukwuemeka Odumegwu Ojukwu University, Anambra state. Three hundred and fifty (350) students participated in the study after informed consent was obtained. The respondents were divided into four groups. Group A consisted of cigarette smokers. Group B was marijuana smokers. Group C was cigarette and marijuana smokers. Group D was control (non-smokers).Questionnaires were distributed to the respondents .ASSESS peak flow meter (RESPIRONICS, U.S.A) was used to measure peak expiratory flow rate (PEFR).The results revealed that cigarette smokers had a mean PEFR of 396±12 LPM. Marijuana smokers had a mean PEFR of 586±16 LPM. Cigarette and marijuana smokers had a mean PEFR of 555±20 LPM. The control (non-smokers) had a mean PEFR of 506±12 LPM. Cigarette smoking significantly decreased PEFR, while marijuana smoking had a positive effect on PEFR.