Anxiety Medication That Causes Weight Loss. Antianxiety medications are sometimes used as a component of anxiety disorder treatment. These medicines alter the levels of various chemical messengers in the brain, which can lead to reduced anxiety and related symptoms. As with any medicine, antianxiety medications can have side effects. Anxiety Medication That Causes Weight Loss Reduced appetite and weight loss are not common side effects with medications used to treat anxiety, but can occur in some people taking certain drugs. Weight loss related to antianxiety medication is usually minimal and rarely a cause of concern. Is This an Emergency? If