How and when to take sildenafil It’s important to take sildenafil as advised by a doctor or pharmacist. You can take it up to 4 hours before you want to have sex. Use the oral syringe provided with your medication to measure and take your dose. Try having sex without alcohol to see if that solves the problem. This increased blood flow can cause an erection. The most common side effects are headaches, feeling sick, indigestion and dizziness. What side effects can this medication cause?AdShop Forhims Online for Great Prices at Store Sale & Clearance. Up To 60% Off Forhims Official SaleK+ visitors in the past month. AdTrust Our Unbiased Reviews and Improve Your Sexual Health in Confidence! Reviews and Comparisons Of The Best Male Enhancement Products. See Which Products WorkK+ visitors in the past month. Hims sells Viagra online at 25, 50, and mg doses. Its side effects are the same as sildenafil, including headaches, skin flushing, and back pain. Pros & cons.