Diphenhydramine

Medically Reviewed by Dr. Harshi Dhingra, M.D.

Last Reviewed: Dec 12, 2022

Warnings

Diphenhydramine Risks, Warnings, and Complications

Pharbedryl (diphenhydramine) can cause serious symptoms in certain groups of people. Ask a doctor or pharmacist about other treatment options for allergy symptoms if you are at high risk of complications from diphenhydramine use.

RISK OF ACCIDENTAL OVERDOSE: Many over-the-counter allergy and cold cough remedies contain diphenhydramine. It is not safe to use more than one medicine with the same or similar ingredients at the same time, as this can lead to an accidental overdose. Read the package labels carefully and follow dosage directions to ensure you do not take more than the recommended maximum dose of diphenhydramine.
SEVERE DROWSINESS: Diphenhydramine causes drowsiness in many people and can affect the ability to think, focus, and react. The chances of this side effect occurring are higher in older adults (age 65 years and above), in those who drink alcohol while taking diphenhydramine, and in those who are taking other medications that also cause drowsiness, such as benzodiazepines, opioid pain relievers, or muscle relaxants. Avoid activities that require alertness, such as driving or operating machinery, until you know how diphenhydramine affects you.
HYPERACTIVITY IN CHILDREN: While Pharbedryl (diphenhydramine) usually causes drowsiness in adults, it can have the opposite effect in children and make them overexcited and hyperactive. Never give diphenhydramine to make a child fall asleep. If your child has difficulty sleeping, talk to their doctor to discuss other options.
WORSENING OF CERTAIN HEALTH CONDITIONS: People with health conditions like glaucoma, diabetes, high blood pressure, urinary conditions, heart problems, thyroid disease, liver disease, kidney disease, and enlarged prostate may have worsening of these conditions if they take diphenhydramine. Give your doctor a complete health history before starting this medicine. Discuss the pros and cons of taking diphenhydramine if you have any medical condition.
RISK FOR PEOPLE WITH PHENYLKETONURIA: People with a condition called phenylketonuria (PKU) should not take excess phenylalanine. The dissolving tablets and some chewable tablets of diphenhydramine contain phenylalanine and are not safe for people with PKU. However, the regular tablets and liquid products are safe to take. Talk to your doctor or pharmacist if you have PKU and are not sure which diphenhydramine products are safe for you.

Precautions Before Starting Diphenhydramine

Tell your healthcare professional if you have ever had an allergic reaction to diphenhydramine, any of the active or inactive ingredients in diphenhydramine products, or any other drugs.

Give your healthcare professional a complete medical history. Diphenhydramine may not be right for people with certain medical conditions such as asthma, emphysema, chronic bronchitis, glaucoma (high pressure in the eye), ulcers, low sodium, prostate enlargement, difficulty urinating, heart disease, high blood pressure, overactive thyroid, or seizures.

Give your health care professional a complete list of all your medicines, including prescription drugs, nonprescription drugs, vitamins, supplements, and herbal products. This can help avoid possible drug interactions.

If you are an older adult (age 65 years and above), talk to your doctor about other safer treatment options. Diphenhydramine is only given to older adults for serious allergic reactions.

Tell your doctor if you are pregnant, could be pregnant, are planning a pregnancy or are breastfeeding. The drug passes into breast milk and can potentially affect the infant.

Precautions During Use of Diphenhydramine

Tell all your doctors you are on Pharbedryl (diphenhydramine) before any surgery, including dental procedures. Keep your regular medical and laboratory appointments. Tell laboratory personnel that you are on diphenhydramine, as this medicine can lead to false test results on certain laboratory tests.

Contact your doctor if you become pregnant while taking diphenhydramine.

Diphenhydramine can make you drowsy and dizzy and affect your ability to focus. Do not drive, operate machinery or do anything else that requires your full attention until you know how this medicine affects you. Remember that drinking alcohol can make such symptoms worse.

Keep in mind that diphenhydramine will help relieve your symptoms but will not cure your condition. If you continue having symptoms after 7 days of taking diphenhydramine, contact your healthcare provider.

Diphenhydramine Drug Interactions

Diphenhydramine can have possible interactions with other drugs. This can affect how well the medications work. Interactions between diphenhydramine and other medicines can also increase the risk of severe adverse effects.

Give your doctor or pharmacist a complete list of your medications, including prescription drugs, nonprescription (over-the-counter) drugs, dietary supplements, and herbal products. Also, tell your healthcare provider if you smoke, drink alcohol, or use recreational drugs because some of these substances can cause serious health complications when used with prescription medicines.

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