In clinical trials, the most common adverse effects of<unk>  use included headache, flushing, indigestion, nasal congestion, and impaired vision, including photophobia and blurred vision. Some<unk>  users have complained of seeing everything tinted blue (cyanopsia). This cyanopsia can be explained because<unk> , while selective for PDE5, does have a minor level of selectivity for PDE6, which is the phosphodiesterse inhibitor found in the retina. Patients thus taking the drug may experience colorvision abnormalities. Some complained of blurriness and loss of peripheral vision. In July 2005, the U.S. Food and Drug Administration (FDA) found that<unk>  could lead to vision impairment in rare cases, and a number of studies have linked<unk>  use with non-arteritic anterior ischemic optic neuropathy. Rare but serious adverse effects found through postmarketing surveillance include prolonged erections, severe low blood pressure, myocardial infarction (heart attack), ventricular arrhythmias, stroke, increased intraocular pressure, and sudden hearing loss. In October 2007, the FDA announced that the labeling for all PDE5 inhibitors, including<unk> , required a more prominent warning of the potential risk of sudden hearing loss. Interactions Care should be exercised by people who are also taking protease inhibitors for the treatment of HIV infection. Protease inhibitors inhibit the metabolism of<unk> , effectively multiplying the plasma levels of<unk> , increasing the incidence and severity of side effects. Those using protease inhibitors are recommended to limit their use of<unk>  to no more than one 25 mg dose every 48 hours. Other drugs that interfere with the metabolism of<unk>  include erythromycin and cimetidine, both of which can also lead to prolonged plasma half-life levels. The use of<unk>  and an α1 blocker (typically prescribed for hypertension or for urologic conditions, such as benign prostatic hypertrophy) at the same time may lead to low blood pressure, but this effect does not occur if they are taken at least 4 hours apart. Contraindications Contraindications include: * Concomitant use of nitric oxide donors, organic nitrites and nitrates, such as: * nitroglycerin * isosorbide mononitrate * isosorbide dinitrate * sodium nitroprusside * alkyl nitrites (commonly known as "poppers") * Concomitant use of soluble guanylyl cyclase stimulators, such as riociguat * Known hypersensitivity to<unk> <unk>  should not be used if sexual activity is inadvisable due to underlying cardiovascular risk factors.
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