I am planing to sell a diet supplement that claims as a sexual
stimulant. My question is what kind of liability I may have if
somebody says it is not working for him/her or has side effect? Is
that enough If I do it through my company with liability insurance?
What kind of coverage should the company received?
Any comments are welcome! Thanks!

Depends on the country you're planning on selling the product in.
Assuming you're selling in the USA, you'd need to get your product
approved by the Food and Drug Administration.

As I understand their website here:
http://www.cfsan.fda.gov/~dms/supplmnt.html, a manufacturer is
required to prove that a product is not harmful. If your product does
have harmful side effects, I think you'd be looking at FDA action as
well as private civil action.

It's also worth noting that it's illegal under FDA regulations to
promote a dietary supplement as a cure for a specific disease or
condition, so you'd have to be careful how you worded you claim that
the supplement was a sexaul stimulatn (e.g. you wouldn't want to claim
it cured impotence).