Is it okay to say "caffeine is an appetite suppressor"? A colleague of mine told me it should be "caffeine is an appetite suppressant". However, I don't know a rule on when to use either of them.
In general, suppressor refers to a person or machine which suppresses something else. Here's one example:
The president reduced murders by 50% and theft by over 90%. He's a great suppressor of crime.
Suppressant on the other hand is more broad, and usually has to do with physiology or pathology. An example might be
The new drug was found to reduce the incidence of the common cold by a factor of 5. Our product is shaping up to be a great disease suppressant.
So, in the case of appetite, suppressant is the more accurate choice. You might also note that the phrase appetite-suppressant is well-established simply as a common phrase, along with disease-suppressant.