We do not treat rash over the phone. We do not recommend use of Benadryl to treat rash.
Why?
Rash can be a manifestion of benign condition (viral infection) as well as serious condition like Anaphylaxis.
Anaphylaxis is defined as a serious allergic reaction that is rapid (minutes to hours) and may cause death. It is known as a "killer" allergy.

Fatal Anaphylaxis can occur following a variety of triggers, although the most common culprits are medications, foods, and insect stings.

Skin symptoms and signs (itchiness, rash, erythema, hives) are present in up to 90% of anaphylactic episodes. And any other system in the body could be affected.

Immediate treatment is with Epinephrine which helps put the breaks on the reaction and sooner it can be administered the better. Delay in epinephrine administration has been associated with biphasic reactions and increased mortality. Biphasic reactions are thought to increase the risk of fatal anaphylaxis.

Benadryl is not the treatment of choice and there is no scientific evidence that it is.

According to the 2010 NIAID Food Allergy Guidelines, the use of antihistamines is the most common reason reported for not using epinephrine and may place a patient at significantly increased risk for progression toward a life-threatening reaction.

To read more about anaphylaxis please visit the following websites:

American Academy of Allergy, Asthma and Immunology
-http://aaaai.execinc.com/videos/Allergy-Videos.asp
Please do not miss video "What is Anaphylaxis"

https://www.aaaai.org/Aaaai/media/MediaLibrary/PDF%20Documents/Practice%20and%20Parameters/2015-Anaphylaxis-PP-Update.pdf

Food allergy and Anaphylaxis Network
www.foodallergy.org