If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. You can take it in a small dose daily or in a larger dose as needed. Tadalafil Cialis. These are commonly prescribed for an enlarged prostate or male pattern baldness. It might work longer if you have mild to moderate erectile dysfunction. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food.This means that 4 hours after you take a dose, your body has cleared half of the dose. In most cases, it takes about 5 half-lives for your body to completely remove a drug from your system. So.  · Maximum dose: 20 mg orally three times a day. Injection: Initial dose: or 10 mg IV bolus three times a day. Comments: No greater efficacy was achieved with oral doses higher than the maximum recommended dose. A 10 mg injection dose is predicted to provide pharmacological effect equivalent to the 20 mg oral dose. swelling in your hands, ankles, or feet; shortness of breath; seizure (convulsions); or. a light-headed feeling, like you might pass out. Common Viagra side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; abnormal vision (blurred vision, changes in color vision) runny or stuffy nose, nosebleeds;.