Is there any side effects very likely to occur from viagra (sildenafil)? and are the side effects permanent?

Depends: There are serious and rarely permanent side effects associated with Viagra (sildenafil). Who gets them depends on underlying medical conditions. Someone young is unlikely to have dangerous side effects, but is also unlikely to need Viagra (sildenafil). Someone young, who has normal circulation, may get a painful erection lasting hours.

Not permanent: Side effects such as headache,l, nasal congestion, and lightheadedness are more common; priapism (sustained, painful erection) and seeing "blue"/vision changes are rare- if not addressed, the latter symptoms can result in permanent damage.