Actually, there is no 'antidote' to the love potion that Oberon gives Puck to use on the Athenian lovers and Titania. What Puck does to sort out the chaos of the situation between the Athenian lovers is that he annoints the eyes of the lovers again and makes sure that when they wake up the first person they will see is the person that they should be with. Note Puck's words at the end of Act III:
When thou wakest,
In the sight
Of thy former lady's eye:
And the country proverb known,
That every man should take his own,
In your waking shall be shown:
Jack shall have Jill;
Nought shall go ill;
The man shall have his mare again, and all shall be well.
So it is that Puck is the character that resolves the hilarious chaos that has caused the Athenian lovers to love each other in hilarious combinations, but it is not an 'antidote' as your question puts it. Rather the magic is used effectively to bring about the happy ending that Oberon desired.