Table of contents
Key Takeaways
- Fish oil and probiotics are among the supplements with the strongest evidence base for pets and are worth considering for most healthy dogs and cats.
- Most pets eating a complete and balanced commercial diet don't need additional vitamins. Supplementing vitamins without a specific deficiency can do more harm than good.
- Joint supplements are widely used but the evidence for their effectiveness is mixed. They're most worth discussing with your vet for older dogs, larger breeds, and pets already showing signs of joint discomfort.
Walk into any pet store and the supplement aisle can feel almost as overwhelming as the one at a human health food shop. Fish oil. Probiotics. Joint support. Multivitamins. Calming chews. Coat boosters. The options are endless, the claims are bold, and it's genuinely hard to know what's worth your money and what's marketing dressed up as wellness.
The honest answer from our vets is that some supplements are genuinely useful for many pets, some are useful for specific pets in specific situations, and some are largely unnecessary for animals eating a balanced diet. Here's how to think through the category and where the evidence actually points.
What counts as a supplement and why the category is so confusing
Supplements occupy a unique regulatory space. Unlike prescription medications, which are rigorously tested and approved by the FDA before they reach the market, supplements are not subject to the same level of pre-market scrutiny. This means that the claims on a supplement label don't have to meet the same evidentiary bar as a drug claim, and the quality and concentration of ingredients can vary significantly between products and brands.
This isn't a reason to dismiss supplements entirely. Several have a genuinely solid evidence base and are worth incorporating into your pet's routine. But it is a reason to approach the category with some healthy skepticism and to prioritize products from manufacturers that invest in quality control and third-party testing.
When our vets recommend a supplement, they're drawing on the available research, their clinical experience, and an assessment of the individual pet's needs. When a supplement company recommends their product, they're selling something. Keeping that distinction in mind is the most useful filter you can apply when navigating the supplement aisle.
Fish oil: one of the most well-supported supplements for pets
If there's one supplement our vets would put in the category of genuinely useful for most pets, it's fish oil. The omega-3 fatty acids found in fish oil, specifically EPA and DHA, have a broad and well-documented range of benefits that apply to dogs and cats as well as humans.
What fish oil actually does
Omega-3 fatty acids are known for their anti-inflammatory properties, and inflammation is at the root of a surprising number of conditions our vets manage. For pets with allergic skin disease, fish oil can help support the skin barrier, reduce the severity of inflammatory reactions, and decrease overall itchiness over time. It's not a replacement for targeted allergy treatment, but as part of a broader management plan it provides real benefit.
Beyond skin health, omega-3s support joint health by helping to reduce inflammatory processes that contribute to arthritis and joint degradation. They support cardiovascular health, brain development in young animals, and cognitive function in older ones. For senior dogs in particular, fish oil is one of the most consistently recommended supplements in veterinary practice.
Choosing a quality fish oil
Not all fish oil products are created equal. Look for products specifically formulated for pets, with clearly stated concentrations of EPA and DHA rather than just total fish oil content. The dose that produces a therapeutic effect is based on the concentration of these specific fatty acids, not the volume of oil. Your vet can help you identify an appropriate dose based on your pet's weight and health status. All Access and Essential members can reach the Modern Animal care team virtually at any hour to ask about specific products or dosing without needing to come in for an appointment.
Probiotics: an emerging area with real promise
Probiotics are live beneficial bacteria that, when ingested in adequate amounts, can positively influence the balance of microorganisms in the gut. In human medicine, interest in the gut microbiome has exploded over the past decade, and veterinary medicine is following closely behind with a growing body of research on how gut health influences overall health in dogs and cats.
What probiotics may help with in pets
The clearest evidence for probiotic use in pets centers on digestive health. Probiotics have been shown to help manage acute diarrhea, reduce the duration and severity of gastrointestinal upset, and support recovery of normal gut flora after antibiotic treatment, which can significantly disrupt the microbial balance in the digestive tract.
Beyond digestion, emerging research suggests that the gut microbiome plays a role in immune function, skin health, and even mood and behavior in dogs. This is an area where the science is still developing, but the direction of the evidence is promising and our vets are paying attention to it.
Choosing a probiotic for your pet
As with fish oil, quality matters considerably in the probiotic category. Look for products that specify the strains of bacteria included, the concentration in colony-forming units (CFUs), and that have been formulated specifically for dogs or cats. Human probiotic products are not necessarily appropriate for pets, as the strains most beneficial for human gut health may differ from those that work best in dogs and cats. Your vet can point you toward products with a stronger evidence base and appropriate formulations for your specific pet.
Vitamins: usually unnecessary for pets on balanced diets
This is the supplement category where our vets are most likely to pump the brakes. For the average dog or cat eating a complete and balanced commercial diet, additional vitamin supplementation is generally not necessary and in some cases can cause harm.
Commercial pet foods that meet AAFCO nutritional standards are formulated to contain every vitamin a healthy pet needs in the appropriate amounts and ratios. Adding a multivitamin on top of a complete diet doesn't provide extra benefit. It can actually create imbalances, particularly with fat-soluble vitamins like A, D, E, and K, which accumulate in the body rather than being excreted when consumed in excess.
There are situations where specific vitamin supplementation is appropriate, including pets on homemade diets that may not be nutritionally complete, pets with specific medical conditions that affect vitamin absorption or increase requirements, and certain life stages where needs are elevated. But for a healthy pet eating a quality commercial food, a daily multivitamin is more likely to be unnecessary expense than meaningful health support.
If you're concerned about whether your pet's diet is providing everything they need nutritionally, that's a conversation worth having with your vet rather than something to address with a supplement off the shelf.
Joint supplements: worth discussing, but with realistic expectations
Joint health is one of the most common reasons pet owners reach for supplements, and it's a genuinely important area of concern. Dogs are living longer than ever, and longer lives combined with the rising rates of pet obesity mean joint disease is something our vets manage with increasing frequency. The desire to do something proactive to protect a pet's joints is completely understandable.
The most commonly used joint supplements are glucosamine and chondroitin, often sold together. These compounds are naturally found in cartilage, and the theory behind supplementing them is that providing the building blocks of cartilage can support joint health and slow its degradation.
What the evidence actually shows
Here's where our vets have to be honest about the limits of what we know. The evidence for oral glucosamine and chondroitin supplements in dogs is mixed. Some studies show modest benefit, particularly in reducing discomfort and improving mobility in dogs with existing arthritis. Others show results that are difficult to distinguish from placebo effects.
One of the underlying uncertainties is whether oral supplementation actually delivers these compounds to the joints in concentrations high enough to make a meaningful difference. The digestive process breaks down many compounds before they reach their target, and it's genuinely unclear how much of what a dog swallows in a joint supplement actually makes it into the joint tissue where it's needed.
This doesn't mean joint supplements are useless. For dogs that show a positive response, they can be a low-risk addition to a broader joint health management plan that includes weight management, appropriate exercise, and when needed, prescription anti-inflammatory medications. But they're not a substitute for those other interventions, and they're not guaranteed to work.
The supplements with the strongest evidence for joint health in dogs are actually omega-3 fatty acids, which brings us back to fish oil as one of the more versatile and well-supported options in the pet supplement category.
Supplements aren't a simple yes or no category, and our vets don't treat them that way. The honest answer, as with most things in veterinary medicine, is that it depends on the pet, the supplement, and what you're trying to address. Fish oil and probiotics have strong enough evidence to be worth considering for most pets. Vitamins are generally unnecessary for pets on balanced diets. Joint supplements may help some dogs and are worth a conversation with your vet, particularly for older or larger breed dogs.
If you're wondering whether a specific supplement is right for your pet, come see us at a Modern Animal clinic or reach out through the app. We're happy to look at what you're considering and give you an honest assessment of whether it's likely to help. Book a visit.
Frequently Asked Questions
Glucosamine and chondroitin supplements are generally considered safe for most dogs with minimal risk of side effects. The bigger question is whether they're effective, and the evidence there is mixed. Some dogs show meaningful improvement in mobility and comfort. Others show little response. They're most worth trying for older dogs, larger breeds, and dogs already showing signs of joint stiffness or discomfort, ideally as part of a broader joint health plan that includes weight management and appropriate exercise.
A few categories warrant caution. Supplements marketed for weight loss in pets have limited evidence and some carry safety concerns. Herbal supplements can interact with medications and are not always safe for pets even when they're considered safe for humans. Supplements containing xylitol, a sweetener toxic to dogs, are an important one to watch for since some chewable supplement products use it as a flavoring agent. Always read ingredient labels carefully and check with your vet before introducing any new supplement, particularly if your pet is on medication or has an existing health condition. All Access and Essential members can reach the Modern Animal care team virtually at any hour with questions about specific products before giving them to their pet.
It's better to use probiotics specifically formulated for dogs or cats. The bacterial strains most beneficial for human gut health may differ from those that work best in pets, and human probiotic products may not be optimized for the digestive environment of a dog or cat. Pet-specific probiotic products with clearly stated strains and CFU concentrations are a more reliable choice. Your vet can recommend specific products with a stronger evidence base for pets.
No. Supplements can be a useful part of a broader joint health management plan, but they are not a substitute for prescription treatments in dogs with significant arthritis or joint disease. Prescription anti-inflammatory medications, weight management, physical therapy, and in some cases surgical intervention are the interventions with the strongest evidence for managing established joint disease. If your dog is showing signs of joint pain including limping, stiffness after rest, reluctance to climb stairs, or reduced activity levels, a vet visit is the right first step rather than reaching for a supplement.
For most pets eating a complete and balanced commercial diet, the answer is no. Quality commercial pet foods are formulated to contain every vitamin a healthy pet needs. Adding a multivitamin on top of a complete diet is unlikely to provide additional benefit and can in some cases create harmful imbalances, particularly with fat-soluble vitamins that accumulate in the body. If your pet is on a homemade diet or has a specific medical condition, talk to your vet about whether targeted supplementation makes sense.
It depends on the supplement and what you're using it for. Fish oil's effects on skin and coat health typically become noticeable over four to eight weeks of consistent use. Probiotics for acute digestive upset may show results within a few days. Joint supplements, if they're going to produce a response, typically require four to eight weeks of consistent use before any change in mobility or comfort becomes apparent. If you've been giving a supplement consistently for eight weeks and aren't seeing any change, that's worth discussing with your vet to determine whether continuing makes sense or whether a different approach is warranted.
Yes. Omega-3 fatty acids provide similar benefits for cats as they do for dogs, including support for skin and coat health, inflammation reduction, and cardiovascular and cognitive support in senior cats. The dosing is different from dogs given the significant difference in body size, and it's worth confirming an appropriate amount with your vet. Look for fish oil products formulated for cats or those that clearly state EPA and DHA concentrations so dosing can be calculated accurately. Some cats are also particular about the taste, so a product designed for palatability in cats can help with acceptance.
Always, ideally. While many supplements are low-risk, some can interact with medications, affect lab results, or be inappropriate for pets with certain health conditions. If your pet is on any prescription medication or has a known health condition, checking with your vet before adding a supplement is especially important. For healthy pets, a quick conversation at your next wellness visit or through virtual care is a good way to get a professional opinion on whether a specific supplement is likely to be useful for your individual animal.