Zac Efron -- Big Smiles in Cocaine Capital of the World

Zac Efron chose an interesting place to escape this month ... jetting off to the mountainous cocaine capital of the world ... but he wasn't there for the skiing.

The above photos were snapped on September 11th in Peru, showing Efron smiling with fans. Sources tell TMZ, Zac flew to the South American country for some mountain climbing, as well as to see the ancient Inca ruins of Machu Picchu.

He was also mobbed by screaming girls a week later at an airport in Cuzco, not far from Machu Picchu.

We're not saying he did or did not partake of the local crop while he was there, but it's still ironic.

TMZ broke the story ... Efron entered rehab twice this year for a serious cocaine problem, most recently in April.

Fun fact: according to the White House drug czar (yeah, we got one of those) Peru overtook Colombia as the world's biggest cocaine exporter in 2011. Now that's what we call winning.