Is it okay to use Benadryl (diphenhydramine) on an 8 month old for rash?

Yes, but why rash? Although not indicated in this age group, i and many pediatrician use Benadryl (diphenhydramine) for younger patients. However, Benadryl (diphenhydramine) is good for allergic causes of rash, but an 8 month old has many, many reasons for rashes, most commonly due to a virus, for which Benadryl (diphenhydramine) would not help. Because a babies immune system is different than an adults, allergic rashes are less common.