However, all medications come with risks. Make sure to get an estimate of when you can expect your prescription in the mail. While each PDE5 inhibitor differs slightly, they’re very similar in treating erectile dysfunction. It’s available in 10 mg and 20 mg tablets. We considered cost, medications available, and whether generic versions were offered to help reduce costs.Read on to learn where to buy Viagra and sildenafil online — plus, more about how Viagra works, its alternatives, and how it compares with other ED medications. Viagra (, 50, 25 and 20mg) takes about 30min to have full effect but cannot be taken with food, as it can diminish the result. The 20mg dose can also be compounded at . There is generic Viagra now. They have 20mg and mg tablets. It’s usually $/bottle depending on your pharmacy.