There are several supplements I take that I am convinced are beneficial to my health and may extend my life. I would like to know if they could be given to my dog, if they would be safe for him. He already takes generic dog vitamins. I wonder if the following supplements would be ok?
Green Tea
Flax Oil
Vitamin C
And maybe when he gets old, DHEA.
Pet stores carry a wide selection of dog vitamins, including the vitamins you mentioned, as well as glucosamine for joint problems, holistic “calming” vitamins and herbs.
The only area of concern is to make sure your dog isn’t getting too much of these supplements. Just like people vitamins, dog vitamins often contain many-times the needed dose of vitamins and minerals, and some of them are undigestible. I don’t know if OD-ing on Vitamin C is dangerous for a dog, but I’ve read that it can cause some health problems in humans, besides being sort of wasteful.
I would suggest that instead of a nutritional supplement, you give your dogs foods which are rich in these substances. My dogs love carrots, so I give them a couple baby carrots every day which gives them Vitamin A in a digestible and nutritious form. (Don’t suddenly give your dog a diet of eggs and carrots, though, 'cause it can upset their tummies. Introduce any new item to their diet slowly.)
Every now and then, I’ll add a little juice or green tea to their drinking water. They seem to really like it, and I’ve never had any upsets by just putting a little bit in there.
There are also treats which contain a little bit of these vitamins sold at pet stores. Alphalpha Bites are one of my dogs’ favorites (and it cuts down on them wanting to eat grass.) That’s probably a better way to go rather than giving them a concentrated pill.
Another thing I have used is Nutri-Stat (made by the Tomlyn company.) It has a huge variety of vitamins, and you can actually keep your pet alive on it if they stop eating because of illness or injury. (It saved my mother’s cat who refused to eat after surgery.) I give it to my eldest dog because it’s hard to keep weight on her. Just a little speck on a bisquit would be plenty to add vitamins to your dog’s diet, and they love the taste.
What Lissa said, regarding quantities of this or that supplement - especially the C. I also have some fairly strong reservations about the green tea for a dog. It would be nice if vetbridge or another vet speaks up.
I would like to recommend you add a tablespoon or so (less for small dogs) of active culture yogurt daily to your dog’s diet. It helps prevent all sorts of gastrointestinal upsets. I give a significant amount of credit to it for my last Dane living past 10 years old (practically ancient for a giant breed). I bought the big (quart?) containers of Dannon plain or vanilla for her.
I’d also recommend a daily vitamin A gelcap (human vitamins) for dogs over 70 pounds (3X weekly for smaller ones). The glucosamine is also a great idea for dogs over 6 years or so old. The carrots and alfalfa treats are fine, if a dog will eat them, but the higher level of A from the gelcap will keep the dog’s coat looking great and - depending on the dog - either eliminate or sharply reduce the amount of shedding.
Another important thing to remember is that dogs are carnivores. While they do eat kibble, if that’s what they’re given, it’s best if most of their diet is meat or meat-derived.
The only thing I really worried about with the green tea is that it has some caffeine in it. I could get decaf tea, but I really wanted to give him a supplement b/c it has much more antioxidants.
Dog dogs need vitamin C? I seem to remeber hearing their bodies make it themselves?
Caffeine is a problem, but I really question whether the antioxidants in green tea are useful for dogs. What Lissa and I have both tried to tell you is that there are other supplements that are likely to have more important and more immediate benefits for a dog. If a vet drops in and says that green tea is good for a dog, I’m willing to back off on this; otherwise, no. You will note she said she gives her dogs a tiny bit occasionally, not regularly.
She said her dogs like carrots. One of my dogs loved apples - the rest just liked them. If you find a fruit or raw veggie with high vitamin, etc., content that your dog likes, there should be no problem giving them a little bit - but probably not every single day. As I said before, they’re carnivores. They have a very short intestine, and so non-meat foods don’t stay with them long enough for them to get that much benefit out of them. I did actual scientific research related to GI problems in dogs, and can’t emphasize too much that people need to realize how very different their digestive systems are from ours. They have a lot in common with us regarding other health and medical problems, but much less where diet and digestion are concerned.
There are good reasons why the “RAW” feeding regimen for dogs has taken hold. I don’t have dogs anymore (disabled), 
so I haven’t had to decide whether I’d feed it. I’m not going to tell you that you should or shouldn’t do that, but if you’re really concerned about your dog’s dietary health, you need to be looking at the whole picture.
A cooked veggie or fruit that retains significant vitamins, etc., is therefore better - especially if it’s chopped, mashed, or otherwise made more easily digestible. There are reasons why the non-meat ingredients in commercial dog foods actually work, in terms of providing nutrition to dogs, and processing is the most important of them. I’d also like to point out that dogs’ ancestors consumption of non-meat foods was almost entirely limited to eating the stomach contents (which were already partly digested) of their prey.
Arthritis is a more common problem in dogs than many people realize. The glucosamine (and chondroitin, and MSM) supplements may be even more helpful for aging dogs than for many aging humans - although I have found them very helpful against osteoarthritis and other problems, not everybody seems to find them all that useful.
Many breeds of dogs do have gastrointestinal problems as they begin to age (some have them even younger). That’s why I think the yogurt is useful, but remember that I recommended about a tablespoon a day for dogs weighing over 70 pounds. For dogs 25-50 pounds, about half that much, and for small breeds, no more than a teaspoon.
One problem with all dogs that weigh less than adult humans, but that is especially bad with small dogs, is that many owners over-estimate how much food period, and especially how much supplements, that their dogs need. Many supplements can cause problems, if used in excess.
That’s why I’m saying don’t give it to him unless a vet says so. You’re the one who listed it at the top. If they even need it, a canine multi-vitamin should have all the vitamin C that most dogs need. I, too, take extra C, but I’d think long and hard before giving it to a dog of mine.
There’s a lot of information available on the web about both health and nutrition for dogs. You might want to do some reading. However, I strongly advise that you get your vet’s advice before making major alterations in your dog’s diet, including supplements that don’t have a track record in dogs, or that haven’t been researched in scientific institutions.
Especially since dogs produce vitamin C all on their own. The idea that if a supplement is good for humans, it must be good for dogs is truly bizarre (and that’s assuming, of course, that the supplement actually has been shown to do something in humans, which is obviously not always the case).
Glucosamine/chondroitin supplementation to treat/prevent joint problems is controversial in dogs as well as in humans. My vet suggested I try it in a Labrador retriever with a knee problem, and it does seem to have improved pain-free activity levels (over a 6-month period). Anecdotal evidence is not much on which to base a course of action, and I’d be hesitant to try this supplement* if there wasn’t already some indication of a problem.
*There are chewable forms (i.e. beef-flavored, yum) that dogs like. Not inexpensive, but cheaper online.