Similarly one may ask, how does Benadryl cause weight gain?
These biological changes result in increased appetite, overeating, slower fat breakdown and weight gain. Antihistamines can also make you feel tired and unmotivated to exercise. Antihistamines that can cause weight gain include: Diphenhydramine (Benadryl and others)
Also, is it bad to take Benadryl every night?
It is not a sleep aid, although many people use it for that purpose. However, even actual over-the-counter sleep aids shouldn't be taken every night. Those who take diphenhydramine for extended periods of time may experience low blood pressure and heart palpitations.
Newer drugs, including cetirizine (Zyrtec), fexofenadine (Allegra) and loratadine (Claritin), are less likely to cause weight gain, according to Long.