Nukitashi - Nukige Mitai na Shima ni Sunderu Watashi wa Dou Surya Ii desu ka? - Vol. 5 Ch. 21 - A Dick Rises and Evil Falls

[King-of-the-End]

Brazilian wandering spider, the poison causes a bunch of nasty effects, but gives you a erection that last for some hours

...and also can kill you if not treated.

You left out the part where there's a chance of impotence after that erection, as well as the fact that said erection will feel painful.