What Causes Drowsiness And Unconsciousness?

                                                  hello sir ,
my name is kartheek dng my final year (BE).sir my father was a patient of lung cancer which he was diagnoised 5yrs back and after getting treated he recovered.Later there was no signs of reoccurence of his disease.But he is suferrer of copd(before detecting cancer) as he was a smoker.Recently doctor prescribed acitrome 2mg which he was usnig from past 6 months and recently after climbing 15-20 steps he sat on a chair telling that his he is having drowsiness and with no sufering i.e., it took less than 2-3min during the time in which he passed away.i guess this may not be the reason i.e., since he climbed the steps he passed away because previously he used to climb steps several times before which are very big and long enough to climb .here what my query is this so what happened is because of using this tablet(ACITROME 2mg).if not what colud be the reason.