Aspirin, acetaminophen and ibuprofen do not have
close similarities based on the classification and molecular structure. Aspirin
is an acetyl derivative and have more than one crystal structure. Ibuprofen,
the ones available as painkiller pills, is actually a racemic form, but like
aspirin, it acts by inhibiting COX enzymes to limit prostaglandin activity.
Acetaminophen is not even an NSAID because it has little anti-inflammatory
activity.
