Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. For sildenafil to work properly, you’ll need to be sexually excited. Sildenafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow easily. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. · Below are details about Viagra dosages commonly used to treat erectile dysfunction (ED).. Viagra form. Viagra comes as tablets that you swallow. Viagra strengths. Viagra tablets are available in. VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Not actual size. Tell all your healthcare providers that you take VIAGRA. If you need emergency medical care for a heart problem, it will be important for your healthcare provider to know when you last took VIAGRA. Do not take VIAGRA if you: take any medicines. Side Effects. 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.