Brittany isn't exactly very bright if she constantly believes everything that her supposed friend, Alana, tells her. She was actually dumb enough to believe Alana's, lies that her neighbor Garner was breaking into her house and, of the list of lies Brittany bought, was poisoning her food. The fact she even believes this in a HOSPITAL, which is a secure place where tampering with food is (almost) impossible shows just how nuts Alana drove her.
Alana is a psychopath who cannot stand having what she wants denied. She wanted Garner's apartment and manipulated Brittany into trying to kill him; having failed to use frivolous lawsuits to get rid of him.
It makes sense Dr. Charles is disturbed by Alana immediately; the woman is worried about MONEY, not her "friend".
Brittany is pretty stupid if she thinks sealed bottles of water and frozen food were poisoned; there would be OBVIOUS signs of tampering. Hell, she probably even thought eating out could still somehow poison her; when Garner doesn't follow her around, at all.