Here's something you may find shocking, but, Benadryl has an effect on multiple organs systems and causes metabolic derangement. The effects are due to the stimulation of the respiratory center, intracellular uncoupling of oxidative phosphorylation, and alteration of platelet function.

Overdosing will give you nausea and vomiting, agitation, hypothermia, lethargy, hyper/hypoglycemia, hyperpnea, tachypnea, tinnitus (ringing in the ears), hemorrhagic gastritis, delirium, stupor, acid-base disturbances, electrolyte imbalance, cerebral edema, convulsions, and cardiovascular collapse, so overdose at your own risk.

If you do overdose, then you have to maintain airway, breathing, and circulation (on your own).