also: ジャンキ
Refers specifically to a person addicted to drugs, matching the original English 'junkie'. Often used in contexts of substance abuse.
He was a junkie when he was young, but now he's rehabilitated.
Extended use for someone obsessed with a particular activity or thing, such as a 'fitness junkie' or 'news junkie'. Colloquial and often used in compound-like expressions.
She's a total fitness junkie and goes to the gym every day.
She's a caffeine junkie who drinks many cups of coffee a day.
He became a drug addict when he was young.
オタク emphasizes obsessive fandom and specialized knowledge (often anime/games), while ジャンキー (sense 2) can attach to almost any activity and stresses compulsive need.
ジャンキー is a casual loanword for an addict, while 廃人 is a harsher, more literary/net-slang term implying the person is completely wrecked by the habit.
adrenaline junkie
Borrowed from English 'junkie', originally meaning a drug addict, later extended to any obsessive enthusiast.