We've done some research and we believe that the sweet spot for posting frequency on Facebook is two posts per day. Buffer, a social media management tool with millions of clients (they know what they're doing), posts to their Facebook page 2-3 times a day. They even created a super helpful infographic about it. You can find the link to their blog at the bottom, under "sources."
Worried that twice a day might be too much? Keep in mind that your posts will likely only be seen as your followers scroll through their news feeds, which means they may or may not see your posts. WebMD posts roughly five times a day, which is not too much for their fans, so posting twice a day is definitely not too frequently. You want to be sure you're still getting in your followers' news feeds. Further, you want to be sure your patients recognize your pharmacy as their source for the latest health information. Posting twice a day shows that you care about providing them with a consistent stream of information to help them stay healthy.