Viagra (Sildenafil) Side Effects

Sildenafil blocks the effects of phosphodiesterases, which are the chemicals that would drain blood from the penis and cause a man to lose an erection. However, while blocking one type of phosphodiesterase can help with erections, blocking another type of phosphodiesterase can cause side effects. Also, any drug has a lengthy list of potential unwanted effects, and erectile dysfunction drugs are no exception.

As noted, Sildenafil is a phosphodiesterase inhibitor and, while generally safe, side effects are possible (even very severe, life-threatening ones).

A phosphodiesterase inhibitor was developed as a blood pressure medication long before it was used to treat erectile dysfunction. In some situations, it can cause dangerously low blood pressure. That is why doctors ask about other medications that can lower blood pressure, especially nitrites/nitrates. Nitrites/Nitrates (found in nitroglycerin and other medications, as well as recreational “poppers”) are so dangerous that their use is an absolute contraindication to prescribing or using Sildenafil.

Another possible side effect of using phosphodiesterase inhibitors can be a change in vision, including blue tinted vision. This happens more with Sildenafil than the other drugs that treat ED. The body contains many phosphodiesterases. Phosphodiesterase 5 is most active in the penis while phosphodiesterase 6 is active in the retina in the eye. Medications rarely are so specific that they will work just on one chemical and not on another very similar chemical. Sildenafil works mostly as an inhibitor of phosphodiesterase 5 but it still inhibits phosphodiesterase 6 to some extent. That is why people, on occasion, get the blue tint when taking Sildenafil or Viagra. Other erectile dysfunction meds affect phosphodiesterase 6 far less.

However, while rare, it is also possible for people using phosphodiesterase inhibitors, including Sildenafil, to suffer complete, permanent blindness in one or both eyes. If this happens, go immediately to an emergency room. It does not matter if it is rare if it happens to you. Be prepared to act immediately if your vision is affected. Sometimes, Sildenafil works too well and an erection can last far longer than it should. If the penis remains engorged (priapism) it can be painful but, more importantly, the tissue and vessels of the penis can be damaged (even permanently). If you have an erection lasting longer than 4 hours, painful or not, seek immediate medical attention.

Also, to one degree or another, all the medicines used to treat ED cause side effects that include: headaches (10%-16%), nasal stuffiness (1%-10%), flushing (5%-12%), dizziness (~3%), and upset stomach (4%-12%). [Drugs 2005;65:1621-1650]

This list is incomplete and the PDR contains a great deal of information and a lengthy list of potential side effects, as well as warnings and contraindications. Please read the list. It is the best way to be prepared and recognize issues that should be addressed emergently.