This drug (and certain other anti-depressants) work by supressing the neurotransmitter serotonin, which happens to be the same neurotransmitter responsible for the mood-altering effects of running. Thus the drug prevents you from gaining the effects of running that you're used to.
This is, I suppose, a way of saying that distance running is a mental disorder, or associated with a mental disorder, and that runners really are "nuts." I don't mean people (millions of them) who run simply to control their weight (and may actually hate running but it works) and those competing to win races and do the training they need (and some of them may not like running, either, and it could be basketball for all they care, as long as they can win). I mean people who cannot win prize money and simply run for the sake of running.