Whitney Houston -- Took Prescription Drug Xanax Before Big Shows

People close to Whitney Houston were aware she was taking Xanax -- and according to our sources, she told them she always took it before big performances ... like the pre-Grammy party she was scheduled to perform at last night.

We're told Whitney believed the Xanax helped calm her nerves before going out in front of people. According to our sources, Whitney claimed to have anxiety and panic attacks and the pills helped her relax before taking the stage.

Our sources say the people inside of Whitney's hotel room when she died were aware Whitney would take Xanax before shows. We're told those people never saw Whitney take any pills, she would just tell them she did. They are unaware how many she would take or if she took any other drugs.

According to our sources, Whitney did not mention taking Xanax yesterday.