Generic versions of sildenafil, vardenafil and tadalafil are available. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. It works for up to 36 hours. If you are taking sildenafil to treat erectile dysfunction, follow your doctor’s directions and the guidelines in this paragraph. Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors.The medication was developed by U.K.-based Futura Medical and will be the first topical OTC treatment for ED that’s available in the U.S. without a prescription, according to a news release. Can I buy generic Viagra without a prescription? No. In most countries, including the United States, Viagra is not available without a prescription. You should also avoid products you can buy online without a prescription that claim either to be generic forms of sildenafil or to contain sildenafil. (Sildenafil is the active drug in Viagra.