The Demi Moore 911 tape will not be very revealing, because we've learned there's gonna be a lot of redacted information ... including the drugs she was using just before the emergency.

Law enforcement sources tell us ... The L.A. City Attorney is recommending that all references to drugs in the 911 conversation between the dispatcher and one of Demi's friends should be erased before being released to the public. The recommendation is based on patient privacy.

TMZ broke the story ... paramedics on scene were told Demi had been doing whip-its ... inhaling nitrous oxide immediately before she fell into semi-consciousness and had seizure-like symptoms.