- Use of prescription drugs (see exceptions listed below) or herbal preparations in the 2 weeks before study commencement. Prior use of medication or herbal preparations in the 4 weeks before study commencement that are intended for weight-loss and/or sold/marketed as weight-loss products or may alter metabolism. Permitted Prescription Drugs: Birth Control, Less than a 7 day short course of antibiotics. Note: Rifampicin is not permitted. Other medicines, such as those for gastroesophageal reflux disease, depression, and Over The Counter analgesics and allergy medications,may be allowed, but will be approved on a case-by-case basis.