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Personally, I find taking the pills two hours before sex to be the optimal time. Half an hour has no effect at all on me. And any alcohol beyond two pints / two glasses of wine has a drastic effect on its efficiency.
I use the tablets for "legit" sex with my girlfriend and if I've taken a dose at night there's a major benefit in the morning too. Those hard-ons that disappear when you get up to pee remain fully functional when there's a a dose in my system.
I found myself taking larger doses to counteract too much drink – as much as one-and-a-half 100mg tablets. But it didn't help that much. I now vary between half a tablet (no alcohol) and a whole one (any alcohol).