But, keep in mind that ED might signal an underlying medical problem, like high blood pressure, heart disease, diabetes, low testosterone, or sleep apnea. Common ones include: Flushing of the skin. Generic versions of sildenafil, vardenafil and tadalafil are available. It works for up to 36 hours. Otherwise, Viagra and Viagra Connect are both taken in the same way: one tablet about an hour before sex — and only one tablet per day. Tadalafil Cialis. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects.According to price-comparison site test1.ru, generic Revatio will cost you as little as $ per pill, depending on where you fill your prescription. That’s compared with . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its . Oral medications, like Viagra and Cialis, which require a prescription and are commonly used to treat ED, can take 30 minutes to take effect, she adds. Injections take less time .