Viagra [Sildenafil Citrate] Overdosage & Contraindications CONTRAINDICATIONS Consistent with its known effects on the nitric oxide/cGMP pathway (see CLINICAL PHARMACOLOGY), VIAGRA was shown to potentiate the hypotensive effects of nitrates, and its administration to patients who are using organic nitrates, either regularly and/or intermittently, in any form is therefore contraindicated. After patients have taken VIAGRA, it is unknown when nitrates, if necessary, can be safely administered. Based on the pharmacokinetic profile of a single 100 mg oral dose given to healthy normal volunteers, the plasma levels of sildenafil at 24 hours post dose are approximately 2 ng/mL (compared to peak plasma levels of approximately 440 ng/mL) (see CLINICAL PHARMACOLOGY: Pharmacokinetics and Metabolism). In the following patients: age >65, hepatic impairment (e.g., cirrhosis), severe renal impairment (e.g., creatinine clearance <30 mL/min), and concomitant use of potent cytochrome P450 3A4 inhibitors (erythromycin), plasma levels of sildenafil at 24 hours post dose have been found to be 3 to 8 times higher than those seen in healthy volunteers. Although plasma levels of sildenafil at 24 hours post dose are much lower than at peak concentration, it is unknown whether nitrates can be safely coadministered at this time point. VIAGRA is contraindicated in patients with a known hypersensitivity to any component of the tablet. OVERDOSAGE In studies with healthy volunteers of single doses up to 800 mg, adverse events were similar to those seen at lower doses but incidence rates were increased. In cases of overdose, standard supportive measures should be adopted as required. Renal dialysis is not expected to accelerate clearance as sildenafil is highly bound to plasma proteins and it is not eliminated in the urine. Information by: healthscout.com

Rather than just cutting and pasting information off another website, I thought I would take a few minutes and write things out in my own words. The contraindications for taking Viagra are 1) hypersensitivity to any of the ingredients of the tablet; and 2) the regular or intermittent use of nitrate-containing medications. The combination of Viagra and nitrates can lead to a dangerous drop in blood pressure, leading to fainting or potentially worse effects. Men with cardiovascular disease or retinitis pigmentosa, a rare genetic eye condition, should talk to their doctor before using Viagra. Also, men who have had a stroke or heart attack within the past 6 months should take great caution when using this medication. [Prescribing information for Viagra (sildenafil citrate): http://www.pfizer.com/download/uspi_viagra.pdf]