The extra liquidity provided by the European Central Bank (ECB) could pose risks to the region's financial stability, according to Mario Draghi, the central bank's president.
Addressing an audience in Frankfurt, he said the new policy measures may entail some risks, but stressed that these risks were "contained."
"Should they emerge, macroprudential policy is best suited to address them," he said.
"Our purchases reduce returns on safer assets," he said. "This encourages investors to shift to riskier, higher-yielding assets. Pension funds, banks and other market participants that we buy securities from are likely to substitute these for other long-term assets, thereby eventually pushing up prices more broadly."
Market reaction since the launch of the bond-buying program on Monday was evidence that asset-purchasing can work for the euro zone, Draghi added. He said this policy was unconventional but "not unorthodox," and stated that it could shield countries from contagion from Greece.