Sildenafil is the non-branded, generic version of Viagra. Tadalafil is the generic version of Cialis. And Vardenafil is the non-branded version of Levitra. Clinically, there shouldn’t be any difference between generic and branded medicines. They contain the same active ingredient, in the same amounts, so should work in exactly the same way.
The main difference between them is that generic medicines are not always made by the company who makes the branded version. The generic tablets may look different to branded tablets, as will the packaging. And usually the non-active ingredients or ‘excipients’ (things like tablet-coating and colouring) are different as well.
For a generic drug to be made available, it has to go through rigorous clinical testing. A generic drug has to work in a comparable way to the branded version.
Right now, there is no generic version of Spedra. The patent on this drug held by the manufacturer is still active, so this is the only company allowed to produce it currently. For this reason, there is no generic version of Spedra in the Generic Trial Pack.