Some Muslims in Fiji Islands drink social drink called kava kava made from a peppery plant root called piper methysticum. The United Nations Office of Drug Control and Crime Prevention has labeled the plant as a narcotic pepper. Please advise whether the drink made from this plant is Haram. Many Muslims consider that because the word Khamr is directed towards wine or alcoholic drinks, drink made from this kava kava root is not covered by either Qur'an or in the Hadith.

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

In simple words, the word Khamr (wine) in Arabic extends to any substance that intoxicates, in whatever form or under whatever name it may appear.
According to Shariah, the following sound Ahadith state that:
"Every intoxicant is Khamr, and every Khamr is Haram". [Reported by Imam Muslim] "Khamr is that which befogs the mind". [Reported by Imams Bukhari and Muslim]
Another Hadith reads: "Prohibition of Khamr is revealed, and Khamr is made from grapes, dates, honey, wheat, and barley. Khamr is (each and every substance that) which befogs the mind". [Reported by Imams Bukhari and Muslim]
Allah knows best.