Guggul is a plant which is known for its active ingredients, the guggulsterones. It has been traditionally used for combatting ailments such as high blood lipids, liver dysfunction and obesity.
Its most well marketed effect, that of being able to increase thyroid activity, does not occur with doses commonly found in supplements. It does, however, exert an anti-cholesterolemic effect (lowers blood cholesterol levels) and is also an anti-inflammatory and anti-oxidant compound.
It is a healthy compound, but not effective for the main means it is marketed for (fat loss).