Better, I now believe, to overreact than to regret lack of action. Tell the school about the mother who crosses the road with her 3 year old trailing behind, tell the pharmacist about the worrying conversation you overheard outside, an addict boasting that he doesn't take his methadone but sells it.
Chat to your local community police, to any police, about the queue of teenagers entering a local house then leaving with with little plastic packets. Ask the sad faced child whether they're all right, the lone toddler whether they can see their mother. Embarrass yourself maybe but don't end up regretting not having spoken out. Don't walk past on the other side.
KateK - I firmly believe that you did the right thing, as do most of us.