MSD

There is no reliable field test to determine how much THC is in the bloodstream, therefore there isn't a way to make a breathalyser test. All you can do is rely on conventional sobriety tests.
I agree that there is legitimate medical usage and that the minimum that should be done is to allow doctors to prescribe it like any other medication. I disagree with your second conclusion. What we do with our bodies is none of the government's business as long as any harm is only done to ourselves. Letting smokers get their weed legally with regulation of the supply chain will move it into the realm of tobacco and alcohol usage. People will buy a pack of joints at the store, or blunts of various potency, just like the grades of pipe tobacco and cigars, and the criminals who use drug traffic to fund their gang wars and criminal enterprises will lose a source of income.
Poison oak does not have discernible medical benefits. Marijuana use for neurological disorders such as MS is, in most cases, more effective and less harmful than conventional drug treatments like interferons, which are known to cause severe liver damage in significant numbers of patients and may cause additional nerve damage.