If a supplement is natural, can it still cause harm?

Yes. Natural describes origin, not safety: many natural substances are pharmacologically active and some are toxic. Fat soluble vitamins accumulate in the body, several plant extracts affect clotting, and various products made for people contain sweeteners dangerous to dogs. The useful question is not whether a product is natural, but how much of which active ingredient it contains.

Does an animal on a complete food need supplements?

Under normal conditions no. A complete food is formulated to cover all known requirements for that species and life stage, and adding vitamins or minerals means exceeding the intended amounts. Supplements make sense when there is a specific goal, for example support in joint disease, and should be chosen for that goal rather than as a general boost.

How long before I can tell whether it is working?

It depends on the category. Joint supplements are assessed in studies after several weeks, often eight to twelve, so judging after seven days makes no sense. Probiotics used in an acute episode work on a scale of days. Setting the trial length before starting avoids both giving up too early and carrying on for years without ever checking.

Can I use supplements made for people?

Not without asking. There are three main problems: the amount per dose is calculated for a body of different size and metabolism, some formulations contain xylitol or other excipients dangerous to dogs, and some fish oil products contain added fat soluble vitamins that can accumulate with prolonged use. Cats require even more caution because of their metabolic peculiarities.

How do I know whether the improvement is real?

By defining what to watch beforehand and writing it down. Veterinary trials show that a substantial share of owners report improvement even when the animal receives a placebo, because attention increases and expectation shapes judgement. Recording concrete behaviours before and during the trial, and reviewing them on a set date, is the only practical way to separate the effect from the wish for one.

Nutrition10 min readUpdated on August 4, 2026

Supplements for dogs and cats: which ones have evidence and which do not

Omega three, joint supplements, probiotics and antioxidants: what the studies show, how to read a claim and when a supplement is simply useless.

Audience: Pet owners
Species: Dog, Cat
Scope: Valid everywhere, European Union, Italy

The pet supplement shelf grows every year, and the promises on it are almost always phrased carefully, because the law does not allow a feed product to claim that it cures anything. That leaves owners in an awkward position: they understand a benefit is being implied, they cannot tell how well that benefit is demonstrated, and in doubt they buy. The problem is not that supplements do not work: some do, backed by controlled clinical trials. The problem is that they are all sold the same way, with the same reassuring design, whether behind them sit ten canine studies or one experiment on cultured cells. Telling those two situations apart is possible and does not require scientific training.

What the law says, and why it matters

In the European Union a supplement for companion animals is not a medicine. It falls under feed legislation, usually as complementary feed or as a product containing authorised feed additives. The practical consequence is that a supplement does not have to demonstrate clinical efficacy before going on sale, while a veterinary medicine must do so through a dossier assessed by an authority.

Two effects follow, visible in any shop. The first is linguistic: packaging carries phrases such as helps to support or promotes wellbeing, which are not vague by accident but sit at the limit of what communication is allowed to say. The second is substantive: two products containing the same substance can differ widely in amount, quality and bioavailability, and nothing obliges anyone to show that a specific formulation produces the effect described in the literature.

Where the evidence is and where it is not

The table below summarises the categories encountered most often, with the level of evidence available from studies in dogs and cats, not from human or laboratory work. The level shown is a summary and does not replace veterinary advice on an individual animal.

Supplement categories and the level of evidence in dog and cat studies
Category	Stated use	Level of evidence
Long chain omega three fatty acids of marine origin	Osteoarthritis, skin, support in kidney disease	Good in canine osteoarthritis, with controlled trials
Probiotics with a defined strain	Acute diarrhoea, gastrointestinal upset	Moderate and tightly tied to the strain studied
Glucosamine and chondroitin	Joints	Weak and inconsistent across available studies
Green lipped mussel extract	Joints	Moderate, with some positive controlled trials
Undenatured type two collagen	Joints	Preliminary but with encouraging results
Antioxidant formulations with medium chain triglycerides	Cognitive decline in the older dog	Moderate, shown mostly as a complete diet
Generic multivitamins in a healthy animal	General wellbeing	No documented benefit, risk of oversupply

Two clarifications make the table usable. The first concerns probiotics: the documented effect belongs to the strain that was studied and does not transfer automatically to another product, even one containing a species with the same name. The second concerns omega three: the evidence relates to long chain fatty acids of marine origin, not to plant oils, because conversion in the body of dogs and cats is inefficient.

When a supplement is useless or harmful

The idea that at worst it will do nothing is the most widespread belief and the least true. Supplements contain active substances, and active substances have a margin beyond which they become a problem.

Vitamins and minerals added on top of a complete food: the ration already covers requirements, and the addition creates excess. Vitamin A and vitamin D accumulate in the body and chronic excess causes bone and liver damage.
Calcium given to a large breed puppy already on a growth food: it interferes with normal skeletal development rather than strengthening it.
Products intended for people given without advice: some are sweetened with xylitol, which is dangerous for dogs, and some fish oils for human use contain added fat soluble vitamins at amounts calculated for an adult person.
Supplements used instead of a diagnosis: a limping dog receives months of joint products while the actual problem needs something else entirely. The time lost is the main harm.
Several products started together: if something improves you do not know what worked, and if an adverse effect appears you do not know what to stop.

How to read a study on a supplement

You do not need to be able to read a scientific paper to judge a claim. Five questions are enough, and they can be put directly to whoever sells the product or to your vet.

Which species was it studied in
Many claims come from studies in rodents, in cultured cells or in people. A result obtained in mice does not transfer to a cat, whose metabolism is profoundly different.
Was there a comparison group
Without a group receiving placebo it is impossible to separate the product's effect from the natural course of the problem, which in joint and gastrointestinal disease spontaneously alternates better and worse periods.
Did the assessors know what was given
In blinded studies neither the owner nor the person measuring knows the allocation. It is an essential precaution, because expectation demonstrably influences judgements about improvement.
What was measured and how
A validated mobility scale, weight bearing measured on a force platform or the number of diarrhoea episodes are measurements. The owner said they were pleased is not.
How long and in how many animals
A study of twelve animals over three weeks provides a hint, not a demonstration. Substances with better evidence have several independent studies reaching the same conclusion.

There is also a manufacturing quality question that concerns the individual product rather than the substance. Independent analyses of supplements on the market have repeatedly found contents different from those declared on the label, sometimes below and sometimes above. Preferring manufacturers that state batch analysis and that give the exact amount of active ingredient per dose is the only practical filter available.

Running a fair trial on a supplement

If you decide to try a product it is worth doing so with a method, otherwise after three months you will not know whether to continue or stop and you will continue out of habit. The method matters above all against a phenomenon well documented in veterinary trials: when owners are given a placebo, a substantial share still report improvement, because they observe more closely and want it to work.

Define what you want to see change
Not a general improvement, but an observable behaviour: gets into the car unaided, rises from the bed without hesitating, takes the stairs in one go, has formed stools five days out of seven.
Measure before you start
Record the starting situation for a week or two. Without that baseline the comparison is made against a memory, and memories adjust to expectations.
Introduce one product at a time
If you also change the food, add a walk or start a painkiller in the same period, the result cannot be attributed. One change at a time, with enough distance between them.
Give the product the time it needs
Joint supplements are assessed after several weeks, often eight to twelve. Probiotics in acute upset work on a scale of days. Deciding this in advance avoids both stopping too early and continuing for years without reason.
Set a date to decide
Put in the calendar the day you will reread your notes and decide whether to continue. Without a date, carrying on becomes the default by inertia.

What to tell your vet

Supplements are the category owners most often forget to mention at appointments, because they are not perceived as treatment. The result is that the vet reasons on an incomplete picture, and in some situations that has concrete consequences.

Before surgery or a procedure under sedation, because of possible effects on clotting and interactions with drugs.
When new medication is started, because some minerals reduce absorption of certain active ingredients when given together.
When blood results come back unexpected, because some substances can influence specific parameters.
In kidney, liver or heart disease, where the mineral or sodium load of a product can matter more than it seems.
In cats, always: metabolic differences from dogs make tolerance to various substances less predictable.

The simplest approach is to keep an up to date list of everything the animal receives, medications and supplements together, with the active ingredient, the amount per dose and the start date. It is the same list needed in an emergency, when there is no time to reconstruct from memory what the animal is taking.

Frequently asked questions

If a supplement is natural, can it still cause harm?: Yes. Natural describes origin, not safety: many natural substances are pharmacologically active and some are toxic. Fat soluble vitamins accumulate in the body, several plant extracts affect clotting, and various products made for people contain sweeteners dangerous to dogs. The useful question is not whether a product is natural, but how much of which active ingredient it contains.
Does an animal on a complete food need supplements?: Under normal conditions no. A complete food is formulated to cover all known requirements for that species and life stage, and adding vitamins or minerals means exceeding the intended amounts. Supplements make sense when there is a specific goal, for example support in joint disease, and should be chosen for that goal rather than as a general boost.
How long before I can tell whether it is working?: It depends on the category. Joint supplements are assessed in studies after several weeks, often eight to twelve, so judging after seven days makes no sense. Probiotics used in an acute episode work on a scale of days. Setting the trial length before starting avoids both giving up too early and carrying on for years without ever checking.
Can I use supplements made for people?: Not without asking. There are three main problems: the amount per dose is calculated for a body of different size and metabolism, some formulations contain xylitol or other excipients dangerous to dogs, and some fish oil products contain added fat soluble vitamins that can accumulate with prolonged use. Cats require even more caution because of their metabolic peculiarities.
How do I know whether the improvement is real?: By defining what to watch beforehand and writing it down. Veterinary trials show that a substantial share of owners report improvement even when the animal receives a placebo, because attention increases and expectation shapes judgement. Recording concrete behaviours before and during the trial, and reviewing them on a set date, is the only practical way to separate the effect from the wish for one.

What to do next

Before your next purchase do two things: look on the label for the exact amount of active ingredient per dose, and ask your vet whether studies exist for that goal in your animal's species. If you decide to try it, write down what you want to see change, record the starting situation for two weeks and put in the calendar the date on which you will decide to continue or stop.