Why report counts mislead

The drug with the most reports in FDA's animal database, counted by active ingredient, is ivermectin + pyrantel pamoate, with 161,643. That number on its own says almost nothing about it. Here is why, and what to look at instead.

The missing denominator

A count is a numerator. To know how often something happens you also need the denominator: how many animals received the product. Sales figures for animal drugs are not public. A monthly chew given to millions of dogs for a decade will gather tens of thousands of reports even if problems are rare; an injectable used a few thousand times a year will gather few even if problems are common. FDA makes the same point: "Commonly used products may have a higher number of reported adverse events due to the higher total number of animals being given the product."

Under-reporting, unevenly

Most events are never reported. Whether one is depends on who noticed, whether they connected it with a drug, and whether they picked up the phone. Striking, sudden events soon after a dose are reported far more readily than slow ones. Owners of companion animals report individually; a problem in a herd may become one report or none.

Publicity

News coverage, social media groups and FDA communications all raise reporting for the products they mention, sometimes for years. A rise in reports after publicity is a rise in reporting. It may or may not reflect a rise in events.

Time on the market

New products are watched closely, and reporting is typically heaviest in the first years after launch, then declines. Comparing a product launched last year with one launched in 2005 compares two different phases.

Who gets the drug

Drugs for arthritis, cancer, heart disease or anaesthesia are given to old or sick animals. Their reports contain more deaths because their patients do, whatever the drug contributes. Parasite preventives are given to healthy young animals, and a large share of their reports say only that the product did not work as expected.

Everything the animal was given is listed

A report lists all recent drugs. Products that are routinely given together appear in each other's reports. A sign on a product's page may relate to a different product on the same report, to an underlying illness, or to nothing in particular.

Posting delay

FDA posts reports quarterly, three to six months after receipt. The newest months on every chart are incomplete, which makes recent trends look like declines.

What is still worth reading

• The mix within one ingredient: which signs are listed, in which species, what kind of reports (health events, lack of effect, product defects).
• Change over time within one ingredient, read with the launch date and any publicity in mind.
• The label. The adverse reactions found in controlled studies are printed on the FDA-approved label, with the denominators this database lacks.

And then a conversation with a veterinarian, who knows the animal.