Nearly all of them? This problem is not caused by some weird, out of line policy of the USA. Basically nowhere wants hard drugs on the streets. And some places that experimented with legalisation rolled it back or changed their approach. Where I live there are still quite a few places with junkie lights, remnants of a hands off approach some decades ago. It led to a central public park becoming completely taken over and turned into a shanty town by addicts. Eventually the approach changed to one of less tolerance, the addicts were moved into rehab or died, and the shanty town was forcibly cleared by police.

I see people on this thread acting as if Mexico's inability to control cartels is a problem exclusively created by America. But those cartels exist elsewhere and America is not the only market for illegal drugs. The issue is wider than that.

That could also be a description of its northern neighbor...

Edit: admittedly, a rather loaded statement. There's a lot more complexity involved, but essentially, it's an analogous situation: corporations and cartels are dictating politics. This is corruption! (Perhaps it's too much to call it a "failed democracy", but it's certainly a "subverted democracy".)

In the context of illegal drugs, Mexico would not be in this state without the US; and vice versa. Is it a parasitic or a symbiotic relationship...?

Mexico has a decaying monopoly on the use of force, not only because the cartels are violent, but also because the police apparatus itself has been corrupted. This would certainly qualify as one of several traditional criteria for a failed state.