Diphenhydramine

(dye-fin-hy-druh-meen)

Trade/other name(s): Benadryl, Diphedryl, and others

Why would this drug be used?

Diphenhydramine is sometimes used to treat nausea, and to stop certain side effects from other anti-nausea medicines. It is also used to help people sleep, and may be used for other purposes. Diphenhydramine is most often used to lessen the effects of allergic reactions. You can buy the pills or liquid over the counter, without a prescription, although injectable forms require a prescription.

How does this drug work?

Diphenhydramine is part of the group of drugs known as antihistamines. These drugs decrease allergic responses by blocking histamine produced by the body. Diphenhydramine has also been found to be slightly effective in preventing the nausea and vomiting caused by chemotherapy. It also can prevent or stop some side effects from other anti-nausea medicines.

Before taking this medicine

Tell your doctor…

• If you are allergic to anything, including medicines, dyes, additives, or foods.
• If you have trouble passing your urine due to prostate enlargement, a blockage in the bladder, or glaucoma. This drug can worsen the symptoms of these problems.
• If you have a blockage in your stomach or intestine. This drug could make it worse.
• If you have asthma, high blood pressure, heart disease, or hyperthyroidism. You may need to be watched more closely while taking this drug.
• If you take tranquilizers (sedatives) such as diazepam (Valium), sleeping medicines, muscle relaxers, pain relievers, or drink alcohol. Diphenhydramine may increase the sleepiness linked to these substances.
• If you have ever had seizures, have an abnormal EEG (brain wave study) or are taking anti-seizure medicines. This drug may raise your risk of seizures.
• If you are pregnant, trying to get pregnant, or if there is any chance of pregnancy. Although there has been no increased risk of birth defects in animals, this drug has not been tested in pregnant women.
• If you are breast-feeding. Some diphenhydramine gets into breast milk and may cause problems for the baby, such as irritability. Diphenhydramine may also decrease the flow of breast milk.
• About any other prescription or over-the-counter medicines you are taking, including vitamins and herbs. In fact, keeping a written list of each of these medicines (including the doses of each and when you take them) with you in case of emergency may help prevent complications if you get sick.

Interactions with other drugs

Substances that make you sleepy, such as alcohol, tranquilizers, opioid pain relievers, anesthetics, muscle relaxants, and sleeping pills, will make you much sleepier and may cause harm when taken with diphenhydramine. If you are on these medicines, your doctor may need to lower your dose.

Antipsychotic medicines (taken for severe mental health problems) may increase your risk of other serious side effects.

Antidepressants known as monoamine oxidase inhibitors (MAOIs, such as isocarboxazid, phenelzine, procarbazine, selegiline, tranylcypromine) or tricyclic antidepressants (such as amitriptyline, clomipramine, desipramine, doxepin, imipramine, and others) may increase your risk of drowsiness and dry mouth if taken with diphenhydramine.

Talk with your doctor or pharmacist before taking any over-the-counter sleeping medicine. Most of them also contain diphenhydramine, and you could end up taking too much.

Check with your doctor, nurse, or pharmacist about whether any of the medicines, vitamins, herbs, and supplements you are taking can cause problems with diphenhydramine.

Interactions with foods

No serious interactions with food are known at this time. Check with your doctor, nurse, or pharmacist about whether foods may be a problem.

Tell all the doctors, dentists, nurses, and pharmacists you visit that you are taking this drug.

How is this drug taken or given?

Diphenhydramine comes in a capsule, pill, or liquid, and can also be given as an injection in a vein or muscle. Dosage, frequency, and duration vary and depend on why you are taking the medicine. Take this drug exactly as directed by your doctor. If you do not understand the instructions, ask your doctor or nurse to explain them to you.

Store the medicine in a tightly closed container away from heat and moisture and out of the reach of children and pets.

Precautions

Diphenhydramine usually causes sleepiness. Do not drive or operate heavy machinery when you are drowsy.

Older adults are more likely to have side effects such as dizziness and confusion when taking diphenhydramine. They are also more likely to feel faint, nervous, restless, or irritable, or have trouble urinating.

Possible side effects

You will probably not have most of the following side effects, but if you have any talk to your doctor or nurse. They can help you understand the side effects and cope with them.

Common

• sedation*
• drowsiness*
• dryness of mouth and other mucous membranes

Less common

• dizziness*
• confusion*
• poor coordination
• excitability (unusual excitement or nervousness)
• low blood pressure, with dizziness or faintness
• discomfort in upper stomach area
• poor appetite

Rare

• blurred vision
• double vision
• ringing in the ears (tinnitus)
• trouble urinating
• inability to fully empty bladder
• frequent urination
• tight chest, wheezing
• thickened mucus in lungs
• rash
• fast heart rate
• serious allergic reaction, with symptoms such as hives (skin welts), itching, dizziness, trouble breathing of swallowing, swelling of face, tongue, lips, or throat
• low white blood cell count with increased risk of infection
• low platelet count with increased risk of bleeding
• low red blood cell count (anemia) with symptoms such as tiredness and dizziness

*See "Precautions" section for more detailed information.

There are some other side effects not listed above that can also occur in some patients. Tell your doctor or nurse if you develop these or any other problems.

FDA approval

Yes – first approved before 1984 (FDA cannot verify dates of drugs approved before 1984).

Disclaimer: This information does not cover all possible uses, actions, precautions, side effects, or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for talking with your doctor, who is familiar with your medical needs.