I am not on messenger, but I can get messages sent thru messenger when I go to FB on my computer (just not on my phone). It tells me who the sender is and the first few words of the message. So if your pharmacist is on FB on his computer, he will have seen that you sent a message.

I would not send another message. I would smile and joke when I saw him and be patient and see what happens. I think you are doing well to not have tried to follow up.