"Tehy didnt do so befroe, and just toppled latin american governments they didnt like"
When the US toppled gov'ts in Latin America we didn't like, we typically took advantage of forces at play already in those countries...we simply helped 'em out a little bit with $$, arms, etc. Hence little or no need for actual US boots on the ground.
Example: Arbenz, in Guatemala. The landowners and Army officers didn't like him. The common folk loved him, but they were badly-educated and largely unarmed (thank you, gun control). We simply assisted the forces who opposed Arbenz in doing what they'd wanted to do anyhow.
There are other examples of US meddling (most of which were more moral), but the above one, in my estimate, was the most tricky to pull off--in other cases, the leader being toppled was just another caudillo, not a freely-elected quasi-populist. In every case, however, there were people who were willing and able to replace the leader we didn't like.
In the case of Colombia, who are we going to replace the government with? The Communist FARC, the ELN (the lesser Commie movement), and the right-wing paramilitaries are drug-dealers too. Everyone in that misbegotten little war sells drugs.
With Peru, are we going to help the Shining Path, a homicidal Maoist group, mount a coup? We spent most of the Cold War helping Peru fight those characters; realpolitik might require a volte face sometime, but I think even the most hard-core drug warriors would find those characters repellent. Plus Shining Path dealt drugs too.
Ecuador? I don't know enough to make a judgement.
Now, we need to figure out what sort of government in these countries would do such a thing. Are there any existing political parties that advocate booting the US out? Is there a Latin American equivalent of the Libertarian Party that wants to end the Drug War? Or does such a scenario require the actual takeover of a state or states by a major drug-dealing group? An analogue could be the Taliban in Afghanistan...they limited drug cultivation to drive up prices for the stockpiles they confiscated from peasants in the name of the "War on Drugs."
If you want a major drug gang to take over a country, a POD would need to be before the destruction of the cartels in the mid-1990s. "Killing Pablo" doesn't work out so well?
In the event of the Medellins taking over Colombia, the US probably would go to war outright. Right or wrongs of the drug war aside, allowing criminal gangs to take over countries would be VERY bad for regional stability. However, if its a domestic political thing, then what? The US isn't a friend of Chavez or Aristide and we didn't intervene on the side of those who oppose those two.