As of this writing, no study has yet been conducted on the direct comparison of the three PDE-5 inhibitors. There are obvious differences, though, and these differences have been studied and discussed thoroughly. With the longer history of Viagra (at least 20 million men treated), they can undoubtedly offer more information and experience than the two newer competitors. German doctors Gleitner and Gressner from the U. Tubingen have studied the differences between the three drugs thoroughly.
Basically, the report showed that all three drugs have about 70% effectiveness, and side effects are generally the same with all three medications. The common side effects include headaches, flushing, and nasal congestion, which are all vasodilatory and are all mild and transient. The only unique side effects are the blue discoloration in vision, seen only in Viagra, and the muscle aches, experienced only with Cialis. Both side effects occur very rarely. None of the drugs may be taken together with nitrates. The only outstanding difference is the long duration of Cialis, the results of which can still be seen 36 hours after a dose.