Grapefruit juice can increase the effects and side-effects of Spedra and therefore should not be taken within 24 hours of taking it.
Alcohol should not be taken at the same time as Spedra as it may lower your blood pressure, making you feel dizzy and/or faint.
Nitrates interact very adversely with all phosphodiesterase type 5 inhibitors like Spedra, Viagra, Cialis, Levitra and sildenafil. They can cause a fatal lowering of blood pressure and should never be used together.
Nitrates include medications like GTN spray, isosorbide mononitrate and nicorandil for angina. Also, the recreational drugs called "poppers" should be avoided.
If you are taking alpha blockers, like doxazocin, prazocin or tamsulosin, then you should leave at least 6 hours between taking your alpha blocker and taking Spedra.