Responses
Just searched "viagra" in the tags and here is the first story that appears, I think it matches your description :
https://www.mangago.me/read-manga/sweet_boy/
WAIT YES THIS IS IT OMG. HOW CAN THIS BES SO SIMPLE. THANKS A LOT
Haha you're welcome, happy to have helped
It was a yaoi manga that's for sure (not manhwa)
I only remember the parts of it, and it had old manga style, not like big chin one but it has the pleasant and old style. Much like this ( https://images.app.goo.gl/TWc6ZLYrWUDdvqL97)
Description: (I'll use uke and seme instead of MC and ML)
The uke is a college student, living with us divorced dad(idk) and his father warns him to not drink a "special" drink inside the refrigerator. But uke does the same and drink it and we all know it was viagra and he starts to have high blood pressure and dizziness so he goes to hospital to seek help. Now the doctor he takes appointment for is the seme, he treats him with meds and since he was a dependent child of his father (some medical insurance or guardian thingy) they need to inform uke's father about this ordeal but uke beggs seme doctor not to because it's embarassing or something. So the seme (some what trashy) ask uke to timely meet him and have sex with him.
(I only remember till this clearly)
Around second chapter he also teaches uke how to perform enema perfectly.
And so on they develop the story.
That's it, that's all I remember. Please let me know!!