The debate is on in Portugal about leaving the euro zone.

It is not the euro as such that causes the problem, but rather the deficit spending rules of the euro zone.

A country outside the euro zone could still use the euro as its currency. Panama and Ecuador use the US dollar as their currency, and the US cannot stop them (and probably would not want to, given the advantages to the US of the use of dollars in commerce elsewhere). Likewise, Portugal, Greece or Spain could leave the euro zone and continue using euros.

However, this would have disadvantages as well as advantages. some of these countries may want inflation to reduce their debt, and if they tried to inflate the euro they would arouse much bigger hostility.