It makes me shudder that people would do headstand without instruction. Let me explain: I have a physical therapist friend who's amazing at what he does. And he begs - all but gets down on his knees and pleads - for me to give up headstand. He says there's no way the neck compression is healthy. My point, that I've been doing it for over 20 years and I do it with meticulous care and perfect alignment, more or less quiets him down, but only barely.
But to just sort of do it....shudder. If I hadn't had a succession of teachers screaming at me to drop my shoulders every 20 secs, to fix my alignment so that everything's totally symmetrical (i.e. well distributed load on the compressed vertebra), and to make me pay much attention to my hand and arm position (otherwise the pose gets cockeyed and the neck is unevenly loaded), I'd have gotten myself into serious trouble by now.
Asana is wonderful for you. But like many things that are strongly wonderful, you've got to learn how to do stuff (and it's well worth the effort). IMO!
Just to give Victor's point its due......if you're in good shape, have very good innate body awareness, and hold headstand a few secs...maybe up to a minute...once in a while, and you're lucky enough not to do something stupid (like tightening your shoulders toward your ears, which is the natural tendency), you probably won't hurt yourself much. But if you've got a regular practice of headstand going, held for a reasonable amount of time and done several times per week, PLEASE GET A TEACHER (me and Victor like Iyengar style, fwiw).