Self-harm is always harmful, and harmful things are not healthy. It's right there in the name.
There's a difference between "physically healthy" and "mentally healthy" and unless you've got both, you're not really healthy.
In the adult world, it's not a judgment or a measurement. It's a spectrum, and you might well be a physically fit drug addict, or a fat social worker.
As a cynical adult... Nobody is completely healthy. And personally, I've never understood the desire to harm myself physically, but I completely understand making decisions that harm me financially, and emotionally.
It's never healthy to harm yourself. I say this as someone who got drunk this evening, and know "drinking" to be harmful to myself. I do it, but it's not healthy.
Dig into why you want to do unhealthy things, and you'll find insight and better questions.