Hangover is caused by excessive drinking.
One of the ideal medicine for its cure is Onion. After attending a cock-tail party in the night, take few pieces of shredded onion with a little lemon-juice and a pinch of salt next day as the first thing in the morning. Many parlours in western countries serve onion-juice in the morning to cure the hangover and jerk the stupor or intoxication out of the drinker’s body.
Other suggested remedies have been enumerated by many previous reviewer in this site, which readers may refer to.
A few are:
Drink lots of water in the morning with a little salt.
During drinking alternate alcoholic and non-alcoholic beverages.
Eat a good breakfast in the morning with eggs, buttered toast and coffee.
Not to drink on empty stomach.
Have a good shower and sleep longer.
Lastly prevention is better than cure- if at all you have to drink, drink within limits.