I've looked at Wikipedia and Wiktionary. I've wondered about this for a long time.
Initially, I though that it was a color code/metaphor for estrogen (female hormones), but then I heard "little pink pills" mentioned in an old song from the 1960s, as well as in an obscure context from the 1980s. None of them explained what they are based on the context.
So what are they? What is a "pink pill", and where was it invented? What does a pill of pink color signify?
I don't think I've ever actually seen a pink pill. They are always either white, "natural" (whatever natural ingredients were put in them), brown or dark-blue or something. It seems to be rather arbitrary.
I wish I could understand what they keep referring to with "pink pills".