The proof I was referring to is in the forms of clinical scientific studies that doctors scientists in the article refer to when explaining the nature of drugs in the body The doctors contacted by The Chronicle agreed that drug residue can remain in fat for a short time but not indefinitely The longest we know that THC the active substance in marijuana stays in the fat is about a month For ecstasy and LSD we re talking about a day or two said Dr Neal Benowitz head of clinical pharmacology at UCSF