The pet supplement shelf has grown faster than the evidence behind it. Most products are sold on a plausible-sounding mechanism, this ingredient does this thing in a cell, rather than on any demonstrated outcome in an actual dog.
So here is an honest sort: what has reasonable support, what is low-risk but unproven, and what can do harm.
The starting position: a healthy dog on a complete, balanced commercial diet does not need supplements. That food is formulated to hit published nutrient profiles, and adding to it can unbalance something that was balanced. Supplements earn a place when there is a specific reason.
Worth buying
Omega-3 fatty acids (fish oil)
The best-supported supplement in dogs, and the one I would pick if limited to one.
Reasonable evidence for: skin and coat quality, joint comfort in arthritic dogs, and general anti-inflammatory support.
What to buy: a marine source, fish body oil, krill oil. Read the EPA and DHA content, not the total oil volume, because that is what carries the effect. A product listing "1,000 mg fish oil" without an EPA/DHA breakdown is telling you nothing useful.
Cautions: it has a mild anti-platelet effect, so raise it with your vet if your dog is on NSAIDs or blood thinners, or has surgery coming up. Fish oil goes rancid, store it cold, buy small bottles, and discard anything that smells wrong. And it is calorie-dense, which matters in an overweight dog.
Cod liver oil is not the same product. It carries substantial vitamin A, which accumulates and becomes toxic at daily doses. Use fish body oil instead.
Targeted probiotics
Reasonable evidence for specific situations, after antibiotics, during and after acute diarrhoea, and for some chronic gut conditions.
Less compelling as a daily supplement for a healthy dog, though it is low risk.
What matters: strain and dose. Products designed for dogs, with named strains and a stated viable count, are a different proposition from a general "probiotic" with neither. Household curd contains live cultures and is a good food, but the strains are not selected for dogs and it is not a substitute when a targeted product is genuinely indicated.
Joint supplements, with realistic expectations
Glucosamine, chondroitin, green-lipped mussel.
Evidence is mixed and modest. Some dogs appear to improve; controlled evidence is weaker than the marketing suggests. They are low-risk and inexpensive, so a trial is reasonable.
Give it a proper trial: eight to twelve weeks, then honestly assess whether anything changed. If nothing has, stop rather than continuing indefinitely out of habit.
They are not painkillers. A dog in genuine arthritic pain needs pain management from a vet, and a joint supplement is at best an adjunct.
Probably unnecessary
Multivitamins, for a dog on complete food. Risk of pushing fat-soluble vitamins into excess.
Coconut oil. Popular, and largely a fat with no specific demonstrated benefit over other fats. Fine in small amounts; not a health intervention.
Apple cider vinegar. Recommended for a remarkable range of things with essentially no supporting evidence in dogs.
Most "immunity boosters" and greens powders. Sold on mechanism, not outcomes.
Turmeric, unless you are targeting joint comfort specifically, and then the dose, the bioavailability and the source all matter more than the fact of giving it.
Can actively harm
Calcium
The most important warning here. Do not give calcium to a growing puppy on a complete diet.
Puppies from weaning to about six months absorb calcium passively and cannot down-regulate intake. Excess is associated with developmental orthopedic disease, osteochondrosis, wobbler syndrome, radius curvus.
Calcium syrup is handed out routinely in India for large-breed and indie puppies. The intuition that bigger bones need more calcium is exactly backwards. Stop the syrup.
The exception: a home-cooked diet genuinely needs a calcium source, because meat is high in phosphorus and low in calcium. That quantity should come from a vet, calculated against the diet, not from a bottle bought on a hunch.
Vitamin A
Accumulates in the body. Usually over-supplied through excessive liver feeding or daily cod liver oil. Chronic excess causes bone and joint problems.
Anything that interacts with medication
Fish oil, turmeric and several herbal preparations have mild anti-platelet effects that stack with NSAIDs and blood thinners. Tell your vet everything your dog is receiving, including things you think of as food rather than medicine.
How to buy sensibly
Products formulated for dogs, where possible. Human formulations may contain xylitol, which is toxic, and are dosed for a different species.
Read the actual active content. EPA and DHA in milligrams. Named probiotic strains with viable counts. "Proprietary blend" means you are not being told.
One at a time. Adding three supplements in a week means you learn nothing about any of them.
Give it eight to twelve weeks, then judge honestly. Most supplements are continued for years on the strength of a hopeful first fortnight.
Tell your vet. Not out of formality, interactions are real, and several supplements affect surgical bleeding.
The bottom line
For most healthy dogs on complete food: nothing is needed.
If you want one thing, make it fish oil, bought on EPA/DHA content, stored cold.
Add a targeted probiotic when there is a gut reason. Try a joint supplement for an arthritic dog with modest expectations and a real stop date.
And if there is calcium syrup in your house for a puppy on complete food, that is the supplement to remove today.
A home-cooked diet is the one situation where supplementation is genuinely required rather than optional, and where guessing at the amounts causes the most harm.