Why should Boots Paracetamol and Codeine Extra Capsules not be taken with monoamine oxidase inhibitors?

Boots Paracetamol and Codeine Extra Capsules should be avoided during MAOI treatment and for two weeks after stopping because opioid interactions may cause serotonin syndrome or severe CNS excitation or depression, including hypertension or hypotension, although the latter interaction has not been documented with codeine.