A spokesman told the BBC the six had been removed from their contracts after claims of misconduct were investigated.
Most were US nationals and included staff and private security contractors, the Wall Street Journal reported.
Use of hashish by local and foreign staff is not unusual, Afghans who have worked at the embassy told the BBC.
The investigation was launched after someone was noticed apparently high on drugs.
The embassy said it took "seriously any incidents of misconduct by Department of State personnel and contractors".
"In this case six personnel who were found to have been using or in possession of prohibited substances were removed from their contracts and no longer work at the US embassy in Kabul."
The US-led coalition has spent billions of dollars on counter-narcotic efforts since launching the war against the Taliban in 2001.
Afghanistan is the world's largest producer of opium, the main ingredient in heroin.
Opium production increased by 43% in the past year, United Nations officials said in October.
Six people working at the US embassy in the Afghan capital, Kabul, have been fired for using or possessing illegal drugs, officials say.