While B12 is a vitamin that can be found in many foods and oral supplements, it is increasingly popular as an injection, because it is thought to aid weight loss. It is often combined with other biochemical substances, including lipotropic compounds. However, their effectiveness remains unproven.
Lipotropics are nutrients that help export fat from the liver. Once that fat is exported, lipotropics also aid in burning it. Certain forms, such as inositol, function very similarly to B vitamins.
B12 is a vitamin that occurs naturally in animal products, is often added to fortified foods, and is also popular in supplement form. It aids metabolism in virtually all of the body's cells, and is also involved in the production of energy and the synthesis of fatty acids.
B12 is purported by its users, and by practitioners at many weight loss clinics, to help speed up the overall metabolic processes and create a greater feeling of overall energy. Because lipotropics directly aid fat breakdown, and are also closely related to B vitamins, when used together, they are thought to intensify each others' effects. They are usually injected separately, but as part of the same overall injection cycle.
Neither compound has been shown in published, peer-reviewed studies to aid weight loss when injected. The FDA has served cease-and-desist letters to several supplement companies who marketed lipotropics as weight loss aids when taken orally, because their effectiveness is unproven.
Neither lipotropic compounds or B12 are regulated as prescription drugs would be, so they are available at many weight loss clinics that may or may not be overseen by a licensed doctor.