How long does it take for Benadryl to start having its full affect on allergies? Need to know because I’m having an allergic reaction right now (lots of red spots and some itchiness on my face, ears, and the outside of my throat and near my ears) and I have deadly allergies and I took Benadryl about 30 minutes ago and I’m not sure if it’s helping. I have moderate throat pain in the inside of my throat, but throat pain is common for me.
When taken by mouth, Benadryl (diphenhydramine) begins to work in about 15 to 30 minutes.
It is important to note that Benadryl also comes in an injectable form as well. The injectable form has a much faster onset of action than oral dosage forms (e.g. capsules, tablets) and generally begins working in as little as 5 to 10 minutes after injecting.
When using Benadryl for allergic reactions, the injectable form would be recommended over oral versions. In fact, Benadryl is recommended in some situations to help stem allergic reactions to vaccines (if they occur) but only the injectable form is, not oral forms (although the oral forms can help relieve itching too).
True allergic emergencies put you at risk of blocked airways due to inflammation and you need medication to work quickly. In addition to emergency medication, medical help should be sought immediately if an allergic reaction occurs.
If a milder allergic reaction is occurring, such as hives and dermatitis, oral Benadryl can be an effective option as it helps to relieve allergy-induced inflammation and itching.
As a final note, when using Benadryl for its sedative effects (i.e. as a sleep aid), most studies report that it will start most making individuals tired within 30 minutes of dosing but it takes around 1 to 3 hours for maximum effects.