It's indefensible to mess with someone's beverage.
Harry is out drinking with a group of friends at the bar and it's Tom's birthday.
Harry wants Tom to really let loose.
Harry coaxes Tom into having several rounds of shots, and they order some bar food.
Harry spikes Tom's drinks with extra alcohol when he's not looking.
Tom had an enjoyable time and feels fine the next day because the food absorbed some alcohol.
Tom has a really severe hangover the following day.