In any case, the selective stimulation of the beta-2 receptor is responsible for the breakdown of fats. Studies have shown that in everyday use over time, the response of receptors to the effects of clenbuterol is reduced. The result in continuously weakened fat burning effects. During these studies, it was also found that the use of ketotifen fumarate (allergy drug) actively increases the sensitivity of the beta-2 receptors. Loss of fat burning effects without the use of ketotifen usually occurs after the second or third week of use. In this case, at least a two-week break is necessary to prelaunch receptors.