Supposedly ketotifen, an antihistamine like benadryl (diphenhydramine), upregulates beta-2 receptors. Clenbuterol is a beta-2 agonist, meaning that at least part of it's fat burning effect is probably from the stimulation of the beta-2 receptor. Over time as your beta-2 receptors are exposed to clenbuterol, they downregulate meaning that the clenbuterol is losing at least some of it's ability to produce it's fat burning effect due to less abundance of beta-2 receptors. So the theory is ketotifen upregulates beta-2 receptors and benadryl is an anti-histamine, then beta-2 receptor upregulation is a property shared by other anti-histamines of which benadryl is one and hence benadryl must also upregulate beta-2 receptors. I'm not sure if there is research regarding beta-2 upregulation specific to benadryl (diphenhydramine), although I have read research specific to ketotifen.