Herbal slimming teas function on a number of different principles and can vary greatly depending on what herbs are used. They generally include thermogenic or lipolytic agents, herbs meant to aid digestion or suppress appetite, or diuretics. They may also include laxatives.
Herbal slimming teas are very similar in their composition to "fat-burning" pills and supplements. They use several mechanisms, which vary greatly from product to product.
Thermogenic stimulants are those that are believed to increase the metabolism, thereby burning calories. Caffeine is the most common of these stimulants. Lipolytic agents are herbs that are believed to cause the body to prioritize burning fat as fuel. Appetite suppressant herbs are intended to reduce the desire for food. Herbs that promote proper digestion are intended to improve nutrient absorption and thus reduce the overall quantity of food needed. Diuretics specifically target water weight; they can therefore lead to a temporary appearance of being slimmer but do nothing to remove actual body fat.
Herbal teas are not regulated by the U.S. Food and Drug Administration, and the claims they make about weight loss may not be backed up by actual medical evidence. This is especially true for diuretics and laxatives, which can cause serious harm if used for an extended period.