Pet Drug Reports

Diphenhydramineadverse event reports that FDA received

Active ingredient

The Green Book lists no brand name for exactly this active ingredient.

The openFDA records give the value "MSK" for the brand name and for the manufacturer name of each drug. FDA says that MSK means "masked". So the site cannot count a report for a brand.

Read this first. FDA says this about the data:

An API dataset query result cannot be used to estimate the risk associated with the product or compare one drug or device product with another. When looking at the results, be aware that there is no certainty that the reported drug or device caused the adverse event. This dataset should not be the sole source of information for clinical decision-making or other assumptions about the safety or effectiveness of a product. The adverse event may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources.

FDA Center for Veterinary Medicine publishes these words. Source: Adverse Event Reports for Animal Drugs and Devices, read on . The data have the reports that FDA received through .

This page is not veterinary advice. For an animal that is ill, speak to a veterinarian.

Contacting a veterinarianHow to report to FDA

From January 1999 to June 2026, FDA received 5,115 adverse event reports that name a drug with diphenhydramine as its active ingredient. Of the 5,115 reports, 4,724 (92.4%) are about dogs. 368 reports are about cats (7.2%).

In January to March 2026, FDA received 112 reports that name this drug. The signs that appear in the largest number of the 5,115 reports are "Vomiting", "Lethargy" and "Diarrhoea". A report is not proof that a drug caused a sign.

Reports that name this drug rose by 18.2% in the 12 months to June 2026, compared with the 12 months before. Reports for all animal drugs fell by 4.6% in the same periods. The newest months can get more reports in a later update. Reports about dogs are 92.4% of the reports that name this drug, and 72.6% of all reports.

Since the last update of the FDA data

The site recorded an update of the FDA data on . FDA changed these counts for January to March 2026:

  • All species: from 102 to 112.
  • Dog: from 86 to 96.

January to March 2026 is the latest closed quarter in the data. In that quarter FDA received 112 reports that name diphenhydramine: 96 dog, 16 cat, 0 horse, 0 other or not given.

All updatesFeed of updates (RSS)

Signs that the reports list

A sign is a coded term from the report. The words in brackets are a plain gloss of the term. The percentage is the share of the 5,115 reports that list the sign. One report can list more than one sign.

The sign that is in the largest number of the reports comes first.

  1. Vomiting29%1,484 reports
  2. Lethargy[unusually tired or inactive]24.3%1,244 reports
  3. Diarrhoea[diarrhea]13%666 reports
  4. Facial swelling10%511 reports
  5. Anaphylaxis[sudden severe allergic-type reaction]9.5%485 reports
  6. Hives9%458 reports
  7. Emesis[vomiting]8.5%435 reports
  8. Anorexia[not eating]6.7%344 reports
  9. Lack of efficacy - NOS[the product did not have the expected effect]6.5%335 reports
  10. Other abnormal test result NOS[abnormal laboratory result, type not specified]6.3%323 reports
  11. Not eating6.2%317 reports
  12. Pale mucous membrane[pale gums]6.2%317 reports
  13. Panting6%307 reports
  14. Fever5.6%286 reports
Show the next 16 signs
  1. Elevated alanine aminotransferase (ALT)[a liver enzyme above the normal range]5.4%275 reports
  2. Decreased appetite5.3%269 reports
  3. Death5.2%267 reports
  4. Death by euthanasia[the animal was euthanized]5.2%266 reports
  5. Pruritus[itching]4.8%248 reports
  6. Behavioural disorder NOS[behaviour change, type not specified]4.6%237 reports
  7. Emesis (multiple)[vomiting more than once]4.5%228 reports
  8. Injection site swelling[swelling where the injection was given]4.4%227 reports
  9. Bloody diarrhoea[diarrhea with blood]3.9%197 reports
  10. Erythema (for urticaria see Immune SOC)3.9%197 reports
  11. Pain NOS[NOS: not otherwise specified]3.8%194 reports
  12. Seizure NOS[seizure, type not specified]3.7%191 reports
  13. Weight loss3.6%183 reports
  14. Anaemia NOS[low red blood cell count]3.5%181 reports
  15. Ataxia[unsteady, uncoordinated movement]3.5%177 reports
  16. Leucocytosis NOS[high white blood cell count]3.4%174 reports

The list shows 30 of 1,604 coded signs. A report is not proof that a drug caused a sign. Download all signs (CSV)

Reports by species

The share is of all 5,115 reports that name diphenhydramine. The other reports give a different species or no species.

Reports for each month, Jul 2021 to Jun 2026

Dog1,547 reports. Largest count in one month: 48 (Apr 2026)
Cat169 reports. Largest count in one month: 10 (Jun 2025)
Horse2 reports

No chart: no month has 5 or more reports.

Other or not given5 reports

No chart: no month has 5 or more reports.

The month is the month in which FDA received the report. Each chart has its own scale. FDA says that a report can take 3 to 6 months to reach the public data. The newest months can get more reports in a later update.

Reports by year

  • Dog
  • Cat
  • Horse
  • Other or not given
Reports that name diphenhydramine, by the year in which FDA received them and by species
YearDogCatHorseOther or not givenAll species
20262212500246
20254125902473
20243283202362
20232723110304
20221871310201
20212811502298
20203472910377
20193082801337
20183051701323
20173391703359
20163922903424
20152971700314
2014204602212
20134432501469
20123472002369
20113510036
200840004
200401001
200310001
200102013
199911002

2026 has the reports that FDA received through Jun 2026. Download the years (CSV)

What the reports record

Outcome at the time of the report

  • Recovered/Normal31.3%1,599
  • Recovered with Sequela[recovered, with a lasting effect]1.9%95
  • Ongoing37%1,893
  • Died5.5%281
  • Euthanized5.2%267
  • Outcome Unknown19.1%976

An outcome in a report is not proof that the drug caused it. The person who sends the report gives the outcome. Each share is of the 5,115 reports that name diphenhydramine.

A report about more than one animal can have more than one outcome.

Kind of report

  • Safety issue84.8%4,335
  • Lack of expected effectiveness4.7%239
  • Both safety and lack of expected effectiveness8.7%446
  • Manufacturing or product defect1.8%93
  • Otherunder 0.1%2

5,085 reports say if the event was serious. 50% of them have the serious flag. The person who sends the report sets the flag.

Show age, weight, breed and route

Age of the animal

  • Under 1 year9.2%471
  • 1 to 3 years29.9%1,529
  • 4 to 7 years28%1,432
  • 8 to 11 years21.1%1,081
  • 12 years and over10.4%532
  • Age not given1.4%70

Weight of the animal

  • Under 5 kg (11 lb)16.9%863
  • 5 to 9.9 kg (11 to 22 lb)25.4%1,298
  • 10 to 24.9 kg (22 to 55 lb)26.1%1,336
  • 25 to 44.9 kg (55 to 99 lb)26.7%1,366
  • 45 kg and over (99 lb and over)2.6%133
  • Weight not given2.3%119

Breed, where the report gives one breed

A breed with many animals has many reports.

Route in the report

  • Oral32.9%1,681
  • Intramuscular6.3%322
  • Subcutaneous2.2%113
  • Parenteral1.6%83
  • Intravenous1.1%55
  • Topical0.2%11

Each bar is on a scale of 0 to 100%.

Approved animal drugs with this active ingredient (FDA Green Book)

This section is reference data. It has no report count.

The openFDA records give the value "MSK" for the brand name and for the manufacturer name of each drug. FDA says that MSK means "masked". So the site cannot count a report for a brand.

The Green Book lists no approved animal drug with this active ingredient. FDA also receives reports for human drugs and compounded drugs that veterinarians prescribe.

Next in the A to Z list

All drugs by active ingredient. Brand names A to Z

FDA pages for this drug

The site checked these addresses on . The links open in a new tab.

Cite this page

Pet Drug Reports. Diphenhydramine: adverse event reports that FDA received. https://petdrugreports.com/drug/diphenhydramine/ Source: FDA Center for Veterinary Medicine, openFDA Animal and Veterinary adverse event reports, export of . Reports received through .

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Where the data come from

The counts come from the openFDA Animal and Veterinary adverse event files, export of 30 Oct 2026. The files have the reports that the FDA Center for Veterinary Medicine received through 26 Jun 2026. The files mask brand and manufacturer names. The site counts a drug by the active ingredients that each record gives, and gives no count to a brand, an application or a company. Ingredient names and the lists of approved products follow the FDA Green Book of 2 Nov 2026. The site does not use the application number that a record has.

The site does not publish report text, lot numbers or details of a reporter. FDA's record is the authority. See how the site counts reports, the data sources and the corrections page.

Questions about these reports

Which brand are these reports about?

The openFDA records give the value "MSK" for the brand name and for the manufacturer name of each drug. FDA says that MSK means "masked". So the site cannot count a report for a brand. The Green Book lists no brand name for exactly this active ingredient.

Do the reports show that the drug caused the signs?

No. FDA states this about the data.

When looking at the results, be aware that there is no certainty that the reported drug or device caused the adverse event.

The adverse event may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources.

FDA Center for Veterinary Medicine. Source: Adverse Event Reports for Animal Drugs and Devices

How many reports did FDA receive in the latest 12 months?

FDA received 501 reports in the 12 months to June 2026. It received 424 reports in the 12 months before. FDA says that a report can take 3 to 6 months to reach the public data. The newest months can get more reports in a later update.

What kinds of report are in the count?

Of the 5,115 reports received from January 1999 to June 2026, 84.8% are about a safety issue only. 13.4% are about a lack of expected effectiveness, alone or with a safety issue. 1.8% are about a defect of the product or its package.