Counterfeiters nowadays use very advanced means to produce their products. It can be difficult, if not impossible, to tell the real product from an imposter without sophisticated equipment.
This slide shows suspect and authentic Viagra tablets. It is hard to see a difference in the tablets with the naked eye, even when the tablets are magnified many times, which they are here. An FDA laboratory with sophisticated technological equipment magnified the imprint (the identification code "VGR 100" pressed into the tablet’s surface) on both the suspect and the authentic Viagra. The laboratory then put the two "G" images on top of one another. When the images are superimposed at this magnification, it is easier to see that the two images are different, suggesting that the imprints were made by different equipment.