Benedryl affecting hpt results?

Di
I recently read that taking Benedryl can produce false negative pregnancy tests. Does anyone know more about this, like how much affects it?
I currently take 100mg of benedryl at night for sleeping (it replaces my ambien since I am ttc...I'm narcoleptic so I have trouble sleeping in the night). I know this is a lot so I'm assuming it can affect my hpt.
Just wondering!