That's because generally people doesn't find any value in liking a pharmacy facebook page. If you're doing a clearance sale & run an ad to promote it, the result might turn out differently.
The thing is that I dont know how to increase the page fans and run successful add campaigns wothout literally spamming everyone, which is something id rather avoid. Also i would to have 500 fans which arw i terested in the products then 1000 friends who liked the page because of me, but are not interested. Anyway, i will write a blog post in future woth specific numbers:budget etc.