Kindly note the following points:(a) In India, the brand name of Sildenafil is Penegra and not Viagra.(b) Sildenafil has an anatomic role and helps in erection. It DOES NOT cause sexual excitement. In fact if there is no desire, sildenafil will not act.(c) Normally the erection will be considerably decreased after ejaculation. Reference to 4 hours is to indicate maximum duration of action of the drug.