Do not use more of it and do not use it more often than your doctor ordered. These medicines are called PDE5 inhibitors. Taking too much Sildenafil does increase the risk of side effects — so do not exceed mg a day. But if it seems to be sticking around, know that effective treatment of erectile dysfunction is possible. It should never be relied upon for specific medical advice.25mg is the minimum dosage of sildenafil approved to treat erectile dysfunction. You might be prescribed sildenafil at this dosage if you have mild ED or any health issues that may increase your risk of side effects from sildenafil. VIAGRA- sildenafil citrate tablet, film coated. (, December). Retrieved from . Viagra’s effects typically last for at least 4 hours. But it can stay in your system for up to 20 hours (close to a day). Avoid taking Viagra at the same time as nitrates such as nitroglycerin (Nitrostat). This combination can cause a dangerous drop in blood pressure. Viagra is available as a lower-cost generic. Manual Health Centre In 30 seconds When taking Viagra for erectile dysfunction, take one 50mg tablet about 1 hour before sex. The effects usually last for around 4 hours, and will only start to work once you’re sexually aroused, so you should take time to relax and be intimate with your partner.