Diphenhydramine Cautions
Objective: Do not use diphenhydramine as the first drug for anaphylaxis. It does not treat shock or airway swelling. It is a poor sedative in older adults. If your protocol includes it, the card sets the amount.
Why this matters
Diphenhydramine is an antihistamine. The patient has hives and wheezing, and it is easy to draw only diphenhydramine. Intramuscular epinephrine is first for anaphylaxis. Antihistamines are add-ons. They do not treat shock or airway swelling. Diphenhydramine is also a poor sedative in older adults. If your protocol includes it, the card sets the amount. This site does not list milligrams.
- If this is anaphylaxis, give epinephrine intramuscularly in the thigh first.
- Diphenhydramine is an optional add-on if your protocol includes it. It is not a reason to skip epinephrine.
- In an older adult, diphenhydramine can cause confusion and falls. It is a poor sedative. Choose a different plan unless the protocol specifically calls for it.
Clinical sequence
- If this is anaphylaxis, give epinephrine intramuscularly in the thigh first. Do not wait for the hives to fade on diphenhydramine.
- Diphenhydramine is an optional add-on per protocol. It is not a reason to skip epinephrine. It does not treat shock or airway swelling.
- Do not use it to sedate an older adult. It can cause confusion and falls. It is a poor sedative, and it is not a default restraint. Painful muscle twisting after some antipsychotics, called dystonia, is a separate use when your protocol includes it.
Teaching points
- Anaphylaxis: epinephrine, oxygen, fluids, and albuterol for wheeze. The antihistamine comes later, if the protocol includes it.
- Dystonia from antipsychotics is a common EMS use. Follow your protocol.
- Anticholinergic poisoning: a diphenhydramine overdose is hot, dry, agitated, and has wide pupils. Care is supportive. See the toxidrome lesson.
- A large ingestion can widen the QRS and prolong the QT by sodium-channel effects. That may be a bicarbonate protocol, not more antihistamine.
Common errors
- Diphenhydramine instead of epinephrine.
- Intramuscular diphenhydramine to calm an older adult.
- Calling the anaphylaxis finished because the hives itch less.
Practice
One-minute check
With a partner: peanut exposure, wheeze, blood pressure 88. Name the first drug, and say where diphenhydramine sits on that call.
Related: Anaphylaxis, Epinephrine.
Sources & Further Study
Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.
- NASEMSO National Model EMS Clinical Guidelines (2022) — U.S. EMS model language. Your protocol still decides.
- AAAAI/ACAAI Anaphylaxis Practice Parameter 2023 — Intramuscular epinephrine first. Antihistamines are add-ons.
Education only. Apply learning within your protocols and medical direction. This is not clinical care guidance.
Check Your Understanding
Questions drawn only from this lesson. After you check, the key is highlighted. Education practice — not a certification exam.