Hi and welcome to HCM
Generally it is recommended not to take viagra
with nitrates since it can cause rapid BP fall. most other antihypertensives can be used safely with viagra but it still doesnt mean that cetain degree of BP fall can happen. You should measure yur bP before you take viagra and it must not be below 110/80.
But considering your case, 80/40 is seriously low BP
and you should not take that risk anymore. You should try with 25mg next time and measure your BP after it. If this happens again you should consider another antihypertensive or avoid viagra forever. Also, it is possible that your low BP was provocated by some other factors such as emotional stress
All in all, be careful,decrease viagra dosage and keep your BP above 110.
Wish you good health. Feel free to ask. Regards