Side Effects or Adverse Reactions
Table 1. Common and Serious Side Effects of Viagra
| Common Side Effects | Frequency |
|---|---|
| Headache | 16% |
| Flushing | 10% |
| Dyspepsia | 7-17% |
| Urinary Tract Infection | 3% |
| Abnormal Vision | 3-11% |
| Diarrhea | 3% |
| Dizziness | 2% |
| Rash | 2% |
| Serious Side Effects | Frequency |
| NAION | Rare |
| Priapism | Rare |
| Hearing Loss | Rare |
| Exfoliative Dermatitis | <2% |
| Angina* | <2% |
| Stroke | <2% |
| Myocardial Infarction* | <2% |
| Severe Hypotension* | <2% |
| Tachycardia | <2% |
| Pulmonary, cerebrovascular, subarachnoid hemorrhage | Rare |
* See warnings and precautions about use with antihypertensives and in patients with cardiovascular disease.
Drug Interactions
Viagra is a vasodilator and it is extensively metabolized by CYP3A4 and 2C9 liver enzymes. Therefore, combining Viagra with other vasodilators will increase the incidence of hypotension, and CYP3A4 or 2C9 inhibitors will increase the blood levels of Viagra.
Table 2. Viagra Drug Interactions
| Drug | Effect | Recommendation |
|---|---|---|
| Nitrates for example, isosorbide, nitroglycerin | Increased hypotension | Contraindicated |
| Alpha adrenergic blockers | Increased hypotension | Caution. Patients should be stable on alpha blocker prior to Viagra administration and the lowest Viagra dose should be used |
| Antihypertensives | Increased hypotension | Caution |
| Ketoconazole, ritonavir, saquinavir, erythromycin, itraconazole, cimetidine, grapefruit juice, other HIV protease inhibitors | Inhibits CYP3A4 metabolism increasing blood levels of Viagra | Avoid combination of Viagra with CYP3A4 inhibitors if possible. See dosage adn administration section for dose recommendations. |
| Rifampin, carbamazepine, phenytoin, phenobarbital, bosentan | Increased metabolism via CYP3A4 liver enzymes with reduced blood levels of Viagra | No recommendation |
Efficacy of Viagra (sildenafil)
- Lasts up to 4 hours and starts working within 30-60 minutes after administration.
- In one study of patients with ED due to diabetes, psychogenic or mixed etiologies, Viagra 25, 50, and 100 mg resulted in 63%, 74%, and 82% of patients reporting improvement in erection, respectively. Only 24% of patients on placebo reported improvement in erection in the same study.
- Cialis versus Viagra: Cialis can be administered daily, works within 15-30 minutes, and lasts 36 hours. Viagra works within 30-60 minutes and lasts about four hours.
Pharmacology (Mechanism of Action)
Viagra is a selective phosphodiesterase type 5 (PDE5) inhibitor. PDE5 is responsible for breaking down cyclic guanyl monophosphate (cGMP), a vasodilator produced in response to nitric oxide that is released due to sexual stimulation in the corpus cavernosum. Inhibition of PDE5 increases cGMP in smooth muscle cells. Cyclic GMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum, causing penile erection. Viagra works in response to sexual stimulation.
