This is a tough one since most places do not allow online pharmacy promotions. You are not going to find a social platform that will specialize in drugs. Legal ones at least.
You can't even use paid advertising on most ad platforms since online pharmacy ads are not allowed on almost all of them.
Personally, I would find something else to promote and drop the online pharmacy venture.
But, if you are head strong on promoting it, I would contact websites that your target market visit and see if they will sell you ad space directly.
I'm pretty sure that was not the response you are looking for, but online pharmacies are considered high risk and not well excepted by almost all ad platforms.