Cialis prescribed for as-needed use is available in dosages of 5 mg, 10 mg, and 20 mg, and most men are initially prescribed 10 mg, so there’s room for adjustment in either direction if necessary. But your physician may choose another starting dosage, and if that’s the case, there’s nothing wrong with asking why. There’s probably a very good reason why your doctor chose to start you with 5 mg or 20 mg, and any good doctor will be more than willing to explain why.