On the Edge - Francine Pascal, Kate William

Series: #40 in Sweet Valley High

Headed for trouble...
Regina Morrow and Bruce Patman have been going together for months. But when beautiful, devious Amy Sutton is paired up with Bruce on a school project, she schemes to steal him away from Regina. Little by little Bruce's resistance to Amy's charm begins to crumble.
Regina is furious when she discovers that Bruce has been seeing Amy behind her back. Hurt and betrayed, she turns to Justin Belson, a troubled senior at Sweet Valley High. Regina's friends are worried. They think Justin and his crowd are bad news--it's rumored that some of them are drug users. Is Regina on a dangerous course? (from Goodreads)

This was such a sad book.

Amy Sutton and Bruce Patman are working on a health class project together about drugs. The more time they spend together, the more Bruce starts lapsing into his playboy self from the beginning of the series, and the more he falls for Amy. They hide it from Regina Morrow, Bruce's girlfriend. When Regina catches them at a BBQ held by the Wakefield twins, she feels betrayed by everyone present and turns to one of troubled kids in school, whose friends are known drug addicts. She goes to a party and tries cocaine. And things don't end well.

There is no subplot.

The end sets up the next book Outcast with molly being treated like she has the plague.