Diphenhydramine injection is an antihistamine used to treat life-threatening allergic reactions (anaphylaxis) along with epinephrine and other treatments. It is also used to relieve symptoms of household allergies, hay fever and the common cold when medication cannot be given by mouth. Common allergy symptoms relieved by antihistamines include rash, itching, watery eyes, itchy eyes/nose/throat, cough, runny nose and sneezing.

It is also used to treat nausea, vomiting and dizziness caused by motion sickness when medication cannot be given by mouth. Diphenhydramine works by blocking a certain natural substance (histamine) that your body makes during an allergic reaction. Its drying effects on such symptoms as watery eyes and runny nose are caused by blocking another natural substance made by your body (acetylcholine).

Diphenhydramine should not be used in newborns or premature infants because of an increased risk of side effects. It should not be used for common cold symptoms in children less than 6 years of age due to the risk of serious side effects, such as slow/shallow breathing. Cough-and-cold products have not been shown to be safe or effective in children younger than 6.

This section contains uses of this drug that are not listed in the approved professional labeling for the drug but that may be prescribed by your health care professional. Use this drug for a condition that is listed in this section only if it has been so prescribed by your health care professional.

This drug may also be used for shaking (tremor) and muscle stiffness caused by Parkinson's disease when medication cannot be given by mouth. It may also be used for side effects of certain psychiatric drugs such as involuntary movements and muscle stiffness (such as EPS).

This medication is given by injection into a vein or into a muscle as directed by your doctor. The dosage is based on your medical condition and response to treatment. Do not use this medication more often than prescribed.

Do not inject this medication into the skin or just under the skin because it can destroy the skin and tissues around the injection site (necrosis).

If you are giving this medication to yourself at home, learn all preparation and usage instructions from your health care professional. Before using, check this product for particles or discoloration. If either is present, do not use the liquid. Learn how to store and discard medical supplies safely.

Tell your doctor if your condition does not get better or if it gets worse.

Drowsiness, dizziness, headache, irritability, stomach upset, blurred vision, decreased coordination, constipation, or dry mouth/nose/throat may occur. These effects may decrease as your body adjusts to the medication. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

To relieve dry mouth, suck on (sugarless) hard candy or ice chips, chew (sugarless) gum, drink water or use a saliva substitute.

To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position.

Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including:

• mental/mood changes (such as restlessness, confusion)
• pounding/irregular heartbeat
• ringing in the ears
• seizure
• difficulty urinating

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• rash
• itching/swelling (especially of the face/tongue/throat)
• severe dizziness
• trouble breathing

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before using diphenhydramine, tell your doctor or pharmacist if you are allergic to it; or to dimenhydrinate; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• breathing problems (such as asthma, emphysema)
• glaucoma
• heart problems
• high blood pressure
• liver disease
• mental/mood disorders
• seizures
• stomach problems (such as ulcers, obstruction)
• overactive thyroid
• difficulty urinating (such as due to enlarged prostate, urinary retention)

This drug may make you dizzy or drowsy or blur your vision. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness or clear vision until you can do it safely. Limit alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Children may be more sensitive to the side effects of this drug. This drug can often cause excitement in young children instead of drowsiness.

Older adults may be more sensitive to the side effects of this drug, especially drowsiness, dizziness, low blood pressure, confusion, constipation, or trouble urinating. Drowsiness, dizziness, and confusion can increase the risk of falling.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

This drug passes into breast milk and may have undesirable effects on a nursing infant. Consult your doctor before breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include:

• antihistamines applied to the skin (such as diphenhydramine cream, ointment, spray)

Tell your doctor or pharmacist if you are taking other products that cause drowsiness such as opioid pain or cough relievers (such as codeine, hydrocodone), alcohol, marijuana (cannabis), drugs for sleep or anxiety (such as alprazolam, lorazepam, zolpidem), muscle relaxants (such as carisoprodol, cyclobenzaprine), or other antihistamines (such as cetirizine, chlorpheniramine).

Check the labels on all your medicines (such as allergy or cough-and-cold products) because they may contain ingredients that cause drowsiness. Ask your pharmacist about using those products safely.

This medication may interfere with certain lab tests (such as allergy skin test), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: severe drowsiness, seizures, widened pupils. In children, mental/mood changes (such as restlessness, irritability, hallucinations) may occur before drowsiness.

Do not share this medication with others.

If you miss a dose, use it as soon you remember. If it is near the time of your next dose, skip the missed dose. Use your next dose at the regular time. Do not double the dose to catch up.

Store at room temperature away from light. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.