I don't know about its safety, but if it does no harm, I've been using toothpaste on my cock when masturbating, it actually intensifies the orgasm a lot. Just wet your cock, put some toothpaste on the head and spread it all around it and start masturbating. The tingling/cool sensation actually makes me feel really good. Do this at your own risk though, I'm not responsible if you hurt yourself with it. Remember to wash after you come because it's definitely not safe to leave toothpaste on your cock for extended long period of time.
Girls can do it, too. Just put some toothpaste on your clit or lips as desire. It may intensify your orgasm over the hill, if you know what I meant. Be careful not to let any get inside your pussy cuz it may cause some infection or some uncomfortable feeling later on. And always remember to wash your pussy after each use.