Every medicine has side effects and Viagra has side effects as well. They may or may not occur, because each person reacts differently to medications. Possibility of side effects is high, if there are associated diseases. The most common side effects are headache, dizziness, flushing, hot flashes, muscle aches, nasal congestion.
It goes without saying that in addition to Viagra there is an alternative drug which can help to increase potency. As a rule, it is a medicine which also has side effects, such as Cialis. To increase potency you should not think about such effects of this drug.