emmm the story about a boy turn into girl after he drink something (i dunno what is it) th...

Dont know about the mangagbut there is anime like this the name was (don't remember )kun and the seven witches

I don't know if this is what you are after but this has a similar plot to what you describe.

Here the boy randomly changed into a girl one day and eventually one of his best friends start to like him.

http://www.mangago.zone/read-manga/mad_cinderella/

I know something like this too but he did not drink anything. He was casted some spell and it turns him into girl. And his partner is someone who is bully. Im not quite sure about the entire story. Its title is cheeky Angel. I think its something related. I hope someone will answer this too. Looking forward to the other's answer

Sounds like this one but it's magical power, not drinking.
It's fun read. http://www.mangago.zone/read-manga/magical_change/ -

Cheeky angel ?

[Marked]

I got nothing but links with girls who woke up and discovered they'd turned into boys/men (if you're interested):

http://www.mangago.zone/read-manga/fujoshi_trapped_in_a_seme_s_perfect_body/

http://www.mangago.zone/read-manga/oyajina/

Idol Pretender?

http://www.mangago.zone/read-manga/idol_pretender/

could be this one: http://www.mangago.zone/read-manga/sekainohate_de_aimashou/

U probably pasted the wrong manga cause this one doesn't involve a boy changing into a girl, still a pretty good yaoi tho

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thanks all ヾ(☆▽☆)