The generic versions are just as effective and much less expensive. From Mayo Clinic to your inbox Sign up for free and stay up to date on research advancements, health tips, current health topics, and expertise on managing health. While each PDE5 inhibitor differs slightly, they’re very similar in treating erectile dysfunction. Generic versions of sildenafil, vardenafil and tadalafil are available. Popular ED medications found online include: Viagra sildenafil : As the first FDA-approved PDE5 inhibitor for ED, sildenafil is taken 30 minutes to four hours before sexual intercourse to help achieve and maintain an erection. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup.This means that 4 hours after you take a dose, your body has cleared half of the dose. In most cases, it takes about 5 half-lives for your body to completely remove a drug from your system. So. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If. With Goodrx, the generic version of Cialis 20 mg (tadalafil) is about $9 for 10 tablets. The brand version is about $ How much is generic Cialis? Cialis and Viagra have both been shown to be safe and effective for erectile dysfunction. The main differences are cost and timing. For occasional use, most people start with Viagra which also.