Are Pink Sheets legal?

Excuse me, could you please clarify for me the legality of Pink Sheets? I've heard conflicting opinions on whether or not trading on these over-the-counter markets is actually allowed. I understand that Pink Sheets are a way for companies to trade their shares without going through the formal listing process of major exchanges, but I'm not entirely sure about the regulations surrounding them. Is it SAFE to invest in stocks listed on Pink Sheets, or should I be wary of potential legal issues?