Sildenafil and tadalafil are the two most widely prescribed treatments for erectile dysfunction. Both have a long track record, and both are now available as affordable generics. The main question is usually not which one is "better", but which one suits the way you live.
Generic sildenafil is typically taken about an hour before sex and works for four to five hours. A high-fat meal can slow it down. Generic tadalafil, on the other hand, can last for up to 36 hours, and food makes little difference. Many men also take a low daily dose of tadalafil so they do not have to plan ahead.
For a side-by-side look, see this plain-English guide walks through timing, food and side effects.
Before you check here start: never take either medicine with nitrate heart medicines or with "poppers". Let your doctor know about any alpha-blockers. The usual side effects are headache, flushing, a blocked nose and indigestion. Get urgent help for an erection lasting more than four hours or any sudden loss of sight or hearing.
Do not chase higher strengths. The licensed doses are 25, 50 or 100 mg for sildenafil and 2.5 to 20 mg for tadalafil. Higher-strength tablets do not reliably work better, but they do make side effects more likely. Stick to the dose on your prescription.
When you buy generic ED tablets online, make sure the strength matches your prescription and keep the leaflet. A valid prescription is required for every order of a prescription medicine. There is more detail here.
This is general information, not medical advice. Always check with your doctor before starting a new medicine.