Other conditions that may disqualify you from an ED prescription include a history of stroke, kidney or liver disease, cancer, diabetes, or a bleeding disorder. The generic versions are just as effective and much less expensive. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. This medicine also works best when you take it an hour before sex on an empty stomach. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or .  · a light-headed feeling, like you might pass out. Common sildenafil side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; .