The Canine Ledger

Compounded Medicines: What They Are and When They're Used

A compounded medicine is mixed to order, often to add flavour or change the strength. In the US, it isn't FDA-approved. What testing has found.

Written by thecanineledger

In a 2024 study, researchers bought capsules of two chemotherapy drugs from nine veterinary compounding pharmacies and measured what was inside. “Of the 68 total samples obtained, 20 (29%) tested outside the FDA-acceptable discrepancy of ±10%” (Bajorek et al., Veterinary and Comparative Oncology 2024, abstract).

While compounding solves real problems, like a dog that won’t take tablets or a dose too small for any tablet, a compounded medicine isn’t checked the way an approved one is. This page explains what compounding is, why vets use it, and what to ask.

1. What compounding is

Mixing a medicine to order. The US Food and Drug Administration (FDA) says compounded animal drugs “are typically made by pharmacists or veterinarians.” They start either from an approved medicine or from a raw ingredient, which the FDA calls a “bulk drug substance”.

Why vets use it. The FDA lists “the need for flavoring (to encourage the patient to take the drug), a different route of administration (such as a capsule instead of a liquid), or a particular dosage that is not approved (such as for a very large or very small animal).”

Simple compounding is routine. The FDA says that “crushing tablets of an FDA-approved medication into a paste or liquid, adding flavor to a medication, or combining two injectable medications in a syringe, are all generally acceptable under certain conditions”.

Our note: This page covers US rules. In the UK, the Veterinary Medicines Directorate (VMD) calls these medicines “extemporaneous preparations”, or “veterinary specials”. It says they “have not been assessed against the same standards of quality, safety (for the target animal, user, consumer and environment) and efficacy as authorised veterinary medicines”. Vets in Great Britain may use them only as the last step of the “cascade”, the order of choices vets follow when no authorised medicine fits. “Extemporaneous preparations carry a higher risk than authorised medicines”. Specialist makers must be authorised and are regularly inspected (VMD, The cascade). We haven’t read the rules in other countries.

2. What “not approved” means

The finished product isn’t FDA-approved. Even when a pharmacy starts with an approved tablet, “the resulting product is not FDA-approved.”

Raw-ingredient compounds haven’t been reviewed. The FDA says these “are not FDA-approved and have not been reviewed by FDA for evidence that they are safe, effective, properly manufactured, accurately labeled, and adequately packaged.”

Problems may not be reported. “Unlike sponsors of approved animal drugs, compounders are not required to report adverse events and product defects to FDA” for medicines made from raw ingredients.

But they can be necessary. When no suitable approved medicine exists, the FDA says a compounded drug “may be a medically necessary treatment.”

3. What testing has found

Chemotherapy capsules. In the 2024 study, drug amounts also varied within batches from the same pharmacy, by up to 51% at some pharmacies. The authors concluded there was “still potential for variability in drug amounts regardless of the pharmacy designation.”

An earlier chemotherapy study. A 2017 study found that potency of one compounded drug “ranged from 71 to 104%”, and of another from 58 to 109% (Burton et al., Veterinary and Comparative Oncology 2017).

An antifungal. A 2018 study measured blood levels in 95 dogs and 20 cats on one antifungal. Levels were much lower with the compounded version than with the branded or generic versions. The authors advised that the compounded form should be avoided, and found the generic a reasonable alternative (Renschler et al., Journal of the American Animal Hospital Association 2018, abstract).

Hand-filled packets. A 2021 study tested a medicine for Cushing’s disease that clinics in Korea repacked into packets. Only 18 of the 44 packets (40.9%) had an acceptable amount of the drug (Nam et al., Journal of Veterinary Internal Medicine 2021). The authors warn this could interfere with how well the treatment works.

Our note: The packet study was in Korea, where clinics often repack medicines into packets. It may matter less elsewhere.

What these studies don’t show. They’re small, and they tested particular drugs and pharmacies. They don’t tell you whether your dog’s compounded medicine is accurate.

Our note: Not every result was poor. In the 2017 study, every sample of the third drug tested stayed within 10% of its labelled strength. In the 2024 study, 48 of the 68 samples (71%) were within the acceptable range. We didn’t find studies designed to show compounded medicines performing well, but that doesn’t mean none exist.

4. What to ask your vet

  • Is there an approved medicine that would work, and why are we compounding instead?
  • Is this made from an approved product, or from a raw ingredient?
  • How should I store it, and when does it expire?
  • Should we check a blood level or recheck sooner, to make sure it’s working?
  • What should I do if my dog doesn’t seem to be responding?

Our note: These questions are ours. The blood-level idea comes from one study of one drug.

→ Buying Your Dog’s Medicines Online, Safely · → Vet Bills, Medicines and Decisions: What You’re Paying For, and What You Can Ask · → Lymphoma: What Chemotherapy Can and Can’t Do

5. What we could not verify

Commonly saidWhat we found
“A compounded drug is the same as the brand”Not necessarily. The FDA says a raw-ingredient compound is “not equivalent” to an approved drug.
“Compounded drugs are illegal”No. US law allows compounding in many circumstances, including flavouring and changing the form.
“Compounded drugs are always inaccurate”Not proven. Some studies found wide variation, but they tested few drugs and pharmacies.
“Compounding is just to save money”Not mainly. The FDA lists flavour, form and unusual doses as common reasons.
“The same rules apply in the UK and Canada”Not in the UK, which has its own rules (see section 1). We haven’t checked Canada.

Sources

Always confirm with your vet before acting on anything on this page.

This is not legal advice. It is our reading of the laws and rules as we best understand them from our research, and no lawyer has reviewed it. We state what the sources say and don't bend them to fit a story. Laws differ by place and change over time, so check the current rules where you live.