if you apply the same reasoning to seatbelt laws, the flaws become obvious
No, not quite: When one drives, one most often drives on shared public roads, and one's action can directly increase risk to others, costs to those who maintain the system, etc. E.g., if you are in an accident and unrestrained, your being unrestrained may place me at greater risk. Your being more seriously injured due to being unrestrained will have greater social costs (financial cost of first response, possible PTSD to first responders, etc.).
One can hardly but drive in a public space, one can hardly ever drive in a private space.
While similar arguments re costs of first response, e.g., can be made about drug use, drug use can be practiced wholly privately, and, with quality drugs and with appropriate delivery systems, including training, risks kept manageable, likely social costs kept low, etc.
The "my body" argument is the central thrust of Roe v Wade and it is the very reason that abortion is no longer illegal. So that argument is absolutely worthy of consideration here.
Indeed, the stronger point is "Don't arrest me because I might commit some other crime." If we applied drug law logic to things like politics, we'd be arresting anyone who became a politician because they might be corrupting our society worse than any drug.
And I don't get your seatbelt argument. If I don't wear my seatbelt, I'm not harming anyone else - even if I get in an accident and get hurt or die due to my lack of wearing a seatbelt. Seatbelt laws are as unconstitutional as are the drug laws IMO. (Although, after reading about seatbelt law constitutionality, I see two decent arguments - 1) the driver can become a deadly projectile in an accident and 2) the driver is more likely to retain control of the vehicle with a seatbelt on. Those are good arguments, but I still don't think they're common enough occurrences that they should limit my personal freedom.)
It's your body, yes, but it's our society. Your reckless endangerment costs money and ties up limited resources.
The fact that prohibition is not an effective strategy makes for a good argument, but "it's my body" rarely does, IMHO.