Taking my turn to answer, viagra principle is that it will increase blood flow, erection comes because of very good blood supply to penis, so viagra suddenly increases blood pumping in body. So it can cause headache to some people or if you already have headache it can increase. But if you have no such conditions 25mg or max 50mg should be fine.
When u take it for the first time, next day u will have headache. But subsequently you won't experience it. However I had experienced a blue tinted vision for couple of hours. It's not advisable if you have high blood pressure . I have so far taken 3 of them once after a gap of 24 hrs, 100 mg each