Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. There are plenty of potential obstacles. Take sildenafil as needed before sexual activity. You should only be treated with one of these products at a time. As part of their research in , Viatris spoke to men about their attitudes to ED. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine.The PDE5-inhibitors available in Ireland are: Viagra (active ingredient – sildenafil) – The oldest and best known PDE-5 inhibitor. You can also buy Sildenafil as a generic. Cialis . What Happened When I Took Viagra Recreationally. 1. Viagra was effective for much longer than four hours. Shutterstock. Viagra comes in three different sizes: 25mg, 50mg, or mg. In the past, I’ve found that 25mg always did the job just fine. However, my journey begins with a bottle of the mg tabs which I’ve chosen to cut into thirds. Tesco does not offer an online Viagra service. Lloyds pharmacies may be able to dispense Viagra in person if you have a prescription. They will not be able to issue a prescription .