I find that I spend too much money when my little people are sick- just trying to guess what they have. Perhaps when they get sick a symptom could be listed?
If you pick up your peep and drop them (sometimes takes multiple times) they will give an indication of what is wrong with them (Itchy, Sore Throat, Headache, etc).
The last spoiler in this post details this and mentions how to know what to give them once you know what their symptoms are.
You may also be interested in a Health Plan for your little people. When you have it your little people get a permanent 75% discount on all medicines!
You can find it in the Coins and More! section of the store. Tap the Store button within the game, scroll down to the Coins and More section, tap it, scroll down some more and you're there. It is an in-app purchase.