In medicine it can be quite difficult to compare medications to each other. Many factors can contribute to this including cost, biases, and limitations of the study groups.
A recent publication compared the effectiveness of medications for erectile dysfunction (Viagra, Levitra, Cialis, Stendra, and Zydena) by reviewing over 100 publications for a total patient population of over 30,000 patients.
Dr. Tang, of the Chinese University of Hong Kong, attempted to see if any of the above medications was superior to the others. His conclusion is that Cialis was better outcomes when compared to the other medications and the side effects were mild. Furthermore, the greatest side effects were flushing and headache, with Cialis having more risks of muscle (especially back) pain.
As noted above, studies such as this can be quite difficult to interpret. Any inherent problems or biases in the original studies will be magnified, and this is not an exact comparison between medications.
That being said, it does lend additional support that these medications TRULY help those men suffering from erectile dysfunction, and offers doctors guidance as to the overall side effects and successes of each medication.