Medicines that block an enzyme called CYP3A4. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow. This medicine works best when you take it on an empty stomach an hour before sex. Do you use any illegal drugs? You can take it in a small dose daily or in a larger dose as needed. The most common side effects of VIAGRA: headache; flushing; upset stomach; abnormal vision, such as changes in color vision such as having a blue color tinge and blurred vision; stuffy or runny nose; back pain; muscle pain; nausea; dizziness; rash.Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil . These medications might not treat your erectile dysfunction immediately. You might need to work with your doctor to find the right medication and dosage for you. Before taking any . How we vet brands and products. If you’re interested in buying Viagra online, five of the best options available are Hims, Roman, Lemonaid, Optum Store, and REX MD. Read on for full details on.