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!!
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/
1. The MC(uke) even tho he is portrayed like a victim/damsel in distress/weak, he is surely not like this. His character really has layers to him like an actual human is. We can say he is more close to reality kind of a character. He is surely two faces, a bitch, a bad friend and a constant good example of "how rich people create their forced imaginary misery", he surely doesn't have anything miserable in his life but is somehow in his own delulu created misery.
2. The ML(seme), like someone already mentioned, he GASLIGHTS, GATEKEEP, GIRLBOSS really well. Given he is full of schemes and had somehow connection to two murders and at a position where say the MC says "him being more kind" might be troublesome. A perfect match to very layered ML.
3. A mysterious dead character which we know a lot about even tho he is mysterious. Like??? Him being a yapper for god's sake
This manhwa could near 9/10 but is not because that would mean it just one point less from perfection so a solid 8/10 does it all the justice taeshin couldn't get.
Well RIP my fav character taeshin you will be missed.