Diphenhydramine: the allergy and sleep medicine that calls for extra care in young children and older adults

For many families, diphenhydramine is the go-to tablet or syrup for a runny nose, an itchy allergic rash or a night when sleep will not come. The drowsiness that helps at bedtime also makes driving risky until you know how it affects you, and alcohol makes it stronger. Children are especially prone to one side effect, excitement, and Health Canada says cough and cold products containing it should not be given to children under six. Older adults are generally advised to avoid it except for serious allergic reactions, so a quick talk with a pharmacist about safer choices can make a real difference.

Everyday exposure: where you meet it and how much

Diphenhydramine is an antihistamine used for allergy and cold symptoms such as sneezing, runny nose, itchy watery eyes and itchy nose or throat; it is also used for motion sickness and, in adults, for trouble sleeping. [1] It comes as tablets, chewable tablets, capsules and liquids, often taken every four to six hours as needed, and it is also sold in combination products. [1] In Canada, diphenhydramine hydrochloride appears in some non-prescription cough and cold medicines, both as an antihistamine and as a cough suppressant. [2]

Known and suspected harms

Evidence labels are editorial summaries, not individual risk predictions: established means recognized under the stated conditions; probable means a likely contribution with uncertainty; possible means limited evidence; disputed means assessors disagree. These labels do not describe how common an effect is.

• Established — drowsiness that impairs driving, worsened by alcohol: diphenhydramine may make you drowsy, so do not drive or operate machinery until you know how it affects you; alcohol adds to the drowsiness. [1]
• Established — common and serious side effects: common side effects include dry mouth, nose and throat, drowsiness, dizziness, muscle weakness and excitement, especially in children. Vision problems and difficult or painful urination are serious side effects that need a doctor's attention. [1]
• Established — not for young children in cough and cold products: MedlinePlus says not to use diphenhydramine in children aged 2 to 5 unless a doctor directs, to use caution at ages 6 to 11, and not to use it to make a child sleepy. [1] Health Canada had non-prescription cough and cold products containing it relabelled in 2009 so they are not used in children under six, citing limited evidence of benefit and reports of misuse, overdose and rare serious side effects. [2]
• Established — generally avoided in older adults: MedlinePlus says diphenhydramine generally should not be used in older adults except to manage serious allergic reactions, because it is not as safe or effective as other options; people 65 or older should discuss risks and benefits with their doctor. [1]
• Established — health conditions that need advice first: tell your doctor if you have lung disease such as emphysema or chronic bronchitis, glaucoma, or trouble urinating from an enlarged prostate, or if you are pregnant or breastfeeding. [1]

Reducing avoidable exposure and harm

Take only the labelled dose, use the measuring device that comes with liquids, never give adult products to children, and match a child's dose to the age chart on the package. [1] If you use a combination product, read its label or ask a pharmacist. [1] Keep it in its original, tightly closed container, up and away and out of children's sight and reach, and dispose of unneeded medicine through a take-back program. [1] In an overdose, call a poison centre; if the person collapses, has a seizure, has trouble breathing or cannot be woken, call emergency services. [1]

What it is

Diphenhydramine (CAS 58-73-1) is an antihistamine; it is commonly sold as diphenhydramine hydrochloride. [1,2,4]

What it does and why it is used

It works by blocking histamine, a substance in the body that causes allergic symptoms. [1]

Regulatory status and health context

Canada

Health Canada lists diphenhydramine hydrochloride among antihistamine and cough-suppressant ingredients in non-prescription cough and cold medicines that should not be given to children under six. [2]

United States

MedlinePlus advises taking diphenhydramine only as directed on the label or by a doctor, and reporting serious side effects to FDA's MedWatch program. [1]

European Union

Diphenhydramine and its salts are on the list of substances prohibited in cosmetic products under the EU Cosmetics Regulation. [3]

Environmental notes

MedlinePlus advises against flushing unneeded medicine and recommends take-back programs. [1] The sources reviewed do not assess diphenhydramine's environmental effects.

Quick facts

• CAS: 58-73-1. [4]
• Type: antihistamine for allergy, cold symptoms, motion sickness and adult insomnia. [1]
• Biggest everyday risk: drowsiness, especially with alcohol or driving. [1]
• Children: not under six in Canadian cough and cold products. [2]
• Older adults: generally avoided except for serious allergic reactions. [1]

Sources

URLs were fetched on 2026-11-10 or 2026-11-11. Source publication dates differ from this draft's review date.

1. MedlinePlus: Diphenhydramine.
2. Health Canada: Concerns about children's medication.
3. EUR-Lex: Regulation (EC) No 1223/2009 on cosmetic products, Annex II.
4. PubChem PUG REST: diphenhydramine synonyms.