Sometime in the 1980's I read a book from the "Young
Adult" section of my local library.  Then years later
I tried to find it again, in order to refer to
something in it, and couldn't find it or remember the
title or author.  My question is: what was that book??

Here is what I remember about the story (to the best
of my ability, considering I was a kid at the time and
this could be as much as 20 years ago, and at 27 I
think I'm suffering from early-onset senility):

Two young girls in a city (not necessarily inner-city
(though maybe), just not rural) in America were best
friends, until one of them moved away.  When they were
teenagers, the girl who had moved either came back to
visit or moved back entirely.  The friend she'd left
had changed quite a lot and seemed to have gone wild,
getting into drugs and running with a bad crowd.  But
in fact she wasn't into drugs, but she was maybe sort
of undercover trying to track down a gang of bad
teenagers who seemed to have killed other teenagers
via drugs, either tainted drugs or an overdose or
something that looked like the deceased had committed
suicide/overdose.  It's possible that in the book,
this girl's brother/sister had died in such a way and
she was out to prove the "real story," although my
memory is iffy on this.  At one point she realizes she
has found the bad guys, and they're ready to knock her
off too, and they prepare to inject something into her
bloodstream that will kill her (for some reason I have
a memory that they're shooting air -- an empty syringe
-- or windex).  I can't remember if they succeed and
kill her or if she escapes / gets rescued.

It sounds like kind of a crap book, I know, but it
bugs me that I can't remember what it was.  Your help
is greatly appreciated!

Hello.

I believe that your book was "Friends for Life" (1983) Ellen Emerson White.

See this review at Allreaders.com:
"This book concerns a teenager, Susan, who is moving back to Boston
with her family after having lived in New York for several years. When
she gets back, she finds her long-time best friend Colleen acting
strangely. Colleen is too thin and jumpy, and she is fixated on a
death which everyone believes was a drug overdose, but that Colleen
insists was murder..."
http://www.allreaders.com/Topics/Info_23864.asp


Also see this reader review at Amazon.com:
"Friends for Life, by Ellen Emerson White, is a very intense mystery
set in Boston. Susan McAllister, a high school senior, moves back to
Boston with her family. However, after only being back a few days her
best friend, all-american Collen Spencer, is found mysteriously dead,
and when the cops find drugs in her locker they assume it was a drug
overdose. Susan knows Collen would never do drugs, but everyone tells
her that she's been gone a couple of years, how does she know Collen
hadn't changed? She believes it is murder, and has good reason to. She
starts investigating the drug scene at school, and as she gets closer
to identifying the killer she puts herself in more danger."
http://www.amazon.com/exec/obidos/tg/detail/-/0380825783/

Available used beginning at $0.24 at Amazon.com:
http://www.amazon.com/gp/product/offer-listing/0380825783/

-----
search terms:
"moving back" "best friend" "drug dealer"

I hope this is the book that you had in mind. If not, please let me
know via the "request clarification" feature.

Fantastic -- quick response, exactly what I was looking for, in spite
of my clearly botched recall of the book's plot details!  Thanks so
much.

Thank you for the tip. 
-juggler