Mastiha (pronounced: mas-tee-ka) is a natural resin that comes from the Mastic tree (basically an evergreen shrub that was first cultivated on the Greek Island of Chios). People in ancient eastern Mediterranean countries commonly chewed (masticated) it to clean their teeth and freshen their breath. Today it can still be found in commercial chewing gums (among other foods) from Greece and other Eastern Mediterranean countries.
I was so surprised to find modern mastiha gum at the grocery store in Athens.
I learned about this fact from: The ELMA gum box.
Apparently they had too much trouble with people disposing of their gum under tables, on elevator buttons, in mailboxes and even inside keyholes. As a result, in 2004 they banned its importation and consequently you can only get chewing gum with a prescription from a doctor or dentist. Supposedly a black market has yet to emerge, but I can’t verify that.
I learned about this fact from: http://en.wikipedia.org/wiki/Chewing_gum_ban_in_Singapore