First off, I'm just a pre-med student with a question that I thought you guys could easily answer: I know that antihistamines like diphenhydramine are CNS depressants. This would mean it decreases heart rate. BUT, I've taken it before and had an increase in heart rate. I did some reading and found out that antihistamines have anti-muscarinic effects, which I presume will only affect the parasympathetic nervous system. Since HR is controlled by the vagus nerve, a decrease in parasymp. activity should increase HR.