Once again check the email headers and you will see how the email scores in spamassassin. However sending an email with just word viagra by itself will not score very highly at all. It is a legitimate word and will only trigger the DRUGS_ERECTILE rule. There needs to be a combination of rules that add up to the trigger point that you have set, by default 5, to start subject tagging.
If you however were to send an email with the word v.i.a.g.r.a ( i.e obfusticated) in it then it will tag it as spam as it now triggers the DRUGS_ERECTILE,DRUGS_ERECTILE_OBFU,GAPPY_SUBJECT,SUBJECT_FUZZY_VPILL rules
Go to
http://spamassassin.apache.org to find out how spamassassin works.
Jon