Each type of molecule corresponds to a specific chemical substance. She also is an advocate for people with mental and/or substance use disorders. Quest admitted being in possession of crystal methamphetamine, a controlled substance. They include depressions due to physical illness, medications, and substance abuse. Some elements can occur as more than a single chemical substance (allotropes). Acetic acid is an example of such a substance. Botulinum toxin is the most poisonous substance known. Laurer also had problems with substance abuse. She went into treatment for substance abuse. Complications may occur due to the substance consumed. Quenching refers to any process which decreases the fluorescence intensity of a given substance. Recording was plagued with problems, many as a result of substance abuse issues. It also has the philosophical advantage of being independent of any particular substance. Individuals suffering from substance abuse display disrupted rhythms. This is in contrast to a substance that is heterogeneous.