A thumb tac is deffinitely dangerous, but It cannot clog your entire digestive tract as a half dollar could.
Don't listen to your buddies, the always seem to know everything (sarcasm). Just remember, pain is your body saying "something is wrong." If you feel any pain or don't see anything by a week from when you took down the coin, go into the doctor... immediatley!
Keep me updated,
All the best.
-Chase