Although the interaction between other protease inhibitors and sildenafil has not been studied, their concomitant use is expected to increase sildenafil levels. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Without physical action to the penis, such as that occurring during sexual intercourse, sildenafil will not work to cause an erection. Your doctor or other health care professional can recommend one for you based on these factors, other health conditions you have and any other medicines you take. If you prefer, you may contact the U. If too much is used, the chance of side effects is increased. Some people may also have ringing in their ears tinnitus or dizziness.Sildenafil oral tablet is available as a brand-name drug and as a generic drug. Brand names: Viagra, Revatio. Sildenafil comes in three forms: an oral tablet, an oral suspension.  · 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. What is Viagra? Viagra is a brand-name prescription drug that’s approved to treat erectile dysfunction (ED). With ED, you’re unable to have or maintain an erection. Viagra is approved for.