Negative interactions between Prozac (and other SSRIs/SNRIs) and amphetamines are classified as "Major".

To quote:

"In general, amphetamines and other sympathomimetic appetite suppressants should not be combined with selective serotonin reuptake inhibitors (SSRIs) or serotonin-norepinephrine reuptake inhibitors (SNRIs)."

This quote is from the "Professional" part of the "Interactions Checker" bit of http://www.drugs.com/

This site is a helpful resource.