Does a breathalyzer test only detect alcohol, or does it detect marijuana too?

Just a rough estimate of blood alcohol level.

Thanks Kendor and Sherasi.
If a person is really high on marijuana, wouldn't that affect their driving, as well? If it's proven that they are high on marijuana, while driving, will they be arrested for that?

Well, if they are stopped and seen to be compromised, and blood testing is done and it is noted to have drug levels in their blood, they will get arrested for drug use and probably driving to endangerment. (not sure of the exact legal terms)

And a darn good thing they don't detect excessive onions to boot!!

LOL Gatman.