 [ Symptoms ] how many mg to make diphenhydramine rx strength [ Disease ] while it is true you can get diphenhydramine benadryl as a prescription the issue is not so much about the dosage amount or number of milligrams as it is about the delivery method prescription diphenhydramine often is delivered intravenously while over the counter diphenhydramine is delivered orally or as a skin cream topically the usual adult dose of oral diphenhydramine is 25mg to 50mg it would be unwise to exceed 50mg of diphenhydramine if for no other reason that it causes extreme drowsiness also people with certain medical conditions should not take diphenhydramine if you have any questions about prescription or over the counter otc medications you can always consult a pharmacist they are the experts when it comes to dosing side effects and every other aspect of medications please do not play backyard chemist by trying to create prescription strength medications from otc versions it is very dangerous hope this helps