Having tried Levitra in 2012, I've recently collected an order for sildenafil from Lloyds, opting for 50mg tabs.
When I went to see Lady Louise XRated, I took half a tab (25mg) as I wanted to check effects. After 35-40 mins, I felt flushed, but like Levitra or viagra, I did not get a thumping headache. The tablet had the desired effect and the onset of flaccidness was much slower after I'd popped, compared to normal.
Next week, I hope to be seeing a 20-year old, so as it's likely to be 1.5hr session, will try a 50mg tablet and report back on whether there was a headache or other effects.