The gf and I have been vaping 3fpm every other weekend since November and my biggest complaint is that although it makes me insatiably horny, I find it impossible to get or sustain an erection. So last night I decided to try some Viagra with the 3fpm. I know it's not generally advised to mix stims and Viagra, so I only took a quarter of a 100mg tablet. It kicked in within half an hour and I was a stiff as a board for about 3 hours. When it wore off i redosed another quarter, which set us up for another 3 hours. No negative effects noted from the combo.
First let me say: that sounds epic. Like crazy epic. How was the orgasm compared to orgasms on other stims?
Now.. Im not judging, just lookin' out for you here.
Just because I care about you

that may end up badly. So this is from the contraindications section of wiki for Viagra (sildenafil):
A few don't apply, but even the heart related one definitely apply. Your blood pressure obviously isn't going to be low while you're ON the 3F-P, but if you withdraw from it, while the viagra is still in your system, that [may] be an issue. Drugs' affect on the kidneys varies from chem to chem, but most kidney-related chems I have done were stimulants.
inadvisable due to
cardiovascular risk factors
Thats probably the biggest worry.
Honestly, Im jealous about your sex session. I needed a good one of those recently and it fell through. And I hate it as much as the next guy when a stim turns out NOT to get your erect.
Just try not to make a habit of Viagra + 3F-P. It would probably be safer to just go with a different chem, 2FA, 2FMA etc... hell if you guys are going at it like that, might as well get a little aPVP or aPHP doesn't seem like it would make the situation worse, then you wouldn't be worrying about the Penis version of a speedball.
Not saying dont do it again, not at all. But try not to make a habit out of it. 3f-P has no scientific data on it really, so who knows exactly what it's doing. But we all like to get FUBAR, so I'm not judging in the least. I would have probably done the same...... and not stopped. Hopefully someone smarter than me can correct me, show me Im wrong, agree with me, tell me I'm a dumbass, send me pictures of osama bin laden. By no means am I a doctor.