What it is
Diphenhydramine is an anti-histamine medication, which means it is an anti-allergy medication. As part of an allergic reaction (including seasonal allergies like pollen), cells called mast cells release histamine, a chemical responsible for some of the sneezing, itching, swelling, and rash that can occur with an allergic reaction. Diphenhydramine helps block the release of histamine.
What it's used for
Diphenhydramine is an excellent medication for swelling and itching due to hives, contact dermatitis, poison ivy, insect bites, bee stings, and allergies.
Diphenhydramine has several other uses because of its side effects. It tends to thicken and to dry nasal and respiratory secretions and mucous, and it is sometimes used (as are other anti-histamines) to treat the runny nose of a cold. While it can help with a runny nose, because it makes the mucous thicker and drier it can actually sometimes make congestion worse, and for this reason I generally do not recommend it to be used for colds. Also for this reason, it should not be given to children with croup and should be used with caution in children with asthma because it can make both conditions worse.
Diphenhydramine also is a sedative and makes people sleepy. It is the main ingredient in over-the-counter adult sleeping pills and in the “nighttime” cold medications. It is a strong enough sedative to cause people to stop breathing when overdosed, so it should be used carefully.
Side effects and safety
- Antihistamines can cause drowsiness (and dangerous levels of sedation if overdosed)
- Some children have the opposite reaction and are stimulated, “revved-up” by antihistamines
- Antihistamines can cause dry mouths and noses (making nose bleeds more likely)
- Diphenhydramine should not be given to children with croup and should be used with caution in children with asthma
Dosing
The dosage of diphenhydramine is 0.5 (1/2) mg per pound of body weight, every 4-6 hours. It should not be given to children less than 6 months of age.
Dosing Chart — given every 4-6 hours
| Weight (pounds) | Dose (mg) | Liquid 12.5 mg per tsp (5 ml) |
Chewable tablets |
Tablets |
| 11-16 (> 6 mos) | 6.25 mg | 1/2 tsp (2.5 ml) | ||
| 17-21 | 9.4 mg | 3/4 tsp (3.75 mL) | ||
| 22-32 | 12.5 mg | 1 tsp (5 ml) | 1 tablet | |
| 33-43 | 18.75 mg | 1 1/2 tsp (7.5 ml) | 1 1/2 tablets | |
| 44-54 | 25 mg | 2 tsp (10 ml) | 2 tablets | 1 tablet |
| 55-65 | 31.25 mg | 2 1/2 tsp (12.5 ml) | 2 1/2 tablets | 1 tablet |
| 66-76 | 37.5 mg | 3 tsp (15 ml) | 3 tablets | 1 1/2 tablets |
| 77-87 | 43.75 mg | 3 1/2 tsp (17.5 mL) | 3 1/2 tablets | 1 1/2 tablets |
| 88 and over | 50 mg | 4 tsp (20 mL) | 4 tablets | 2 tablets |
