I feel two ways about this: in theory and in principle, I believe adolescents are basically adults and should have a lot more rights than they currently have, attaining them gradually throughout adolescence rather than having none on their 17th birthday and then being slapped with all of them on their 18th.
As a mom of young adults who were recently teens, however, I'm aware that teenagers can be both ignorant and full of bravado. Sometimes they do not fully comprehend why a particular medical treatment is necessary, or what the consequences of refusing it might be.
I found that when my kids were being medically noncompliant and all my pleading was merely making them more stubborn and obstinate, they would listen to a doctor (or a nurse, or any medical personnel), and would then comply with whatever treatment was being recommended.
If that hadn't been the case, however, and even doctors had been able to persuade them to comply with treatment... then I don't know. It's a really tough situation, much more complex than it seems, on the surface.
One would hope that when it's a potentially life-threatening matter, teens could be somehow persuaded to follow medical recommendations. I'd hate to see them forced. Whether I'd support forcing them would really depend on what their reasons were for refusing.
With things like metastatic cancer, it's a different matter.
Many adults refuse treatment as well, as the odds of a complete cure are extremely negligible.
But if it's something like a teen who has cut himself on rusty metal refusing a tetanus shot... I'd almost be inclined to advocate force, if they were refusing only out of defiance, bravado, or a belief that they were immortal and nothing bad could ever happen to them.