The She's The Man star is "showing increasingly alarming signs of delusional behaviour", members of her West Hollywood gym told TMZ.
One gym-goer said Amanda came to the facility last Thursday "looking dazed and began speaking with herself".
Another local, who claimed to have seen the actress dozens of times in the past few months, said she was clearly "deteriorating", randomly stopping mid-workout and "laughing hysterically for no reason".
TMZ poked further into Amanda's neighbourhood to back up the claims.
One neighbour said he saw Amanda on various occasions "having long conversations with inanimate objects". According to another neighbour, those in her apartment block are aware of her strange behaviour but don't want to get involved in her high-profile dramas with the law.
A close friend recently told Radar Online that "Amanda is refusing to go to rehab or seek any form of psychiatric treatment or diagnosis".
"Amanda's completely gone off the grid. She won't speak with her manager, her agent, her publicist; she's not returning anybody's phone calls," the friend told Radar.
The Hairspray actress was pulled over by police today to have her car impounded, E! News reports.
Amanda is due in court on September 21 to address issues related to her bail following her April DUI charge. She lost her license in August after two alleged hit-and-runs, which will be discussed in court on September 27.
Do you have a celebrity story that should be published on TheFIX? Taken a sneaky pic of a star that you want to share with the world? Send your photos, tip-offs and gossip to email@example.com