To access cheats, open your phone at the top of the screen, click settings then toggle cheats to on.
Para acceder a los trucos, abre tu teléfono en la parte superior de la pantalla, haz clic en Configuración y luego activa los trucos.
So far the serum has been used on a total of 3 occasions throughout the game and each time with a purpose to the plot.
First - Jackson uses it with the skater girls after his and the serum's introduction to the game. This scene explains both what the serum actually does and shows immediately what an asshat Jackson is. Also that even without the serum, those two girls are already promiscuous enough and given the chance, probably would have cheated on their boyfriends anyway, as you find out for sure later while working at the Gym.
Second - Although we don't technically see it happening, Lucas uses it on Abigail during the party. The purpose of this scene is to establish the primary potential conflict between you and Lucas. And as we later find out by talking with Abigail - Even though she tells you to your face she would never normally have gone behind Christine's back with Lucas, she follows up by clearly hinting that she has no problem using sex as a means to keep you on her side. So it's not unreasonable to assume that serum and best friend's boyfriend or not, chances are she would've let Lucas have a go regardless, given the right circumstance.
Thirdly - Jackson uses up the last of his serum on Claire at the pub. This scene has two purposes. It sets the tone for your relationship with Claire, by either giving it a head start or nipping it right in the bud.
It's second purpose is that you just saw him use the very last drop of it. And while that may not seem too important right now, it's actually a big deal for the upcoming plot in Episode 3.
As for the MC not being able to compete, that's simply not true.
Now, this question is more of a rhetorical one, so please don't think I'm expecting you to answer it, but how many sexual partners have you had in your life that have never been with anyone else before, ever? The point of the question is to illustrate that just because a sexual partner already has experience with other people, doesn't mean you couldn't get with them yourself.
That's just life.
We've already seen that the MC stands more than a decent chance with all of the girls that have been dosed by Lucas and Jackson (with the exception of Lana, and that's simply because it hasn't been included yet).
And for the record, we're nowhere near the ending - happy or otherwise. But what I will say is that I've already written one for both Lucas and Jackson. Their's most certainly are not.