Several things to point out in this thread, not feeling "better", doesn't mean they aren't working. If your cells are dividing and multiplying, replacing old dying cells, you don't "feel" it, you don't feel it working better or worse but in the long term you might find yourself healthier in years to come as your body is, for want of a better word "regenerated" better over the years. Keep in mind the healthier you are the more efficiently your body replaces said dying cells and to a certain degree you could certainly be doing yourself a good thing in the long run.
Cod liver oil in excess is VERY dangerous due to the vitamin D overdose posibilities, its why omega fish oils and not cod liver oils are what you should take if you want omega oils, its also why you aren't recommended to eat real fish more than a few times a week, the vit D, not anything else. Omega's are used in pretty much every function in the body, not to mention your brain is made up mostly of omega fats and you'll find short term and long term benifits of eating a diet with a decent amount of omega's in. They'll help metabolism, heart functions, liver/kidney function, joints, just about anything else aswell.
But you need to keep this in mind for the people talking about a balanced diet meaning none of this is needed. Farmland is FAR less nutrient enriched than it used to be because we grow far more food far more quickly, it quite literally has less beneficial vitamins in that it used to. One of the key arguments with vitamins is that you won't absorb much from them anyway, this is true to a certain degree, whats also true is the vitamin content in vegetables is NOT all in easily absorbable form, infact cooked vegetables make many of the vitamins easier to absorb but again not fully, raw veg doesn't do a whole hell of a lot of good for you.
But more realistically we simply survive on a lot less vitamins than we've been led to believe so the "lessened" numbers from a vitamin tab aren't as bad as first thought. THen the reason vitamins from tablets aren't all easily absorbed, we didn't know what to take. If you take loads of calcium to strengthen your bones, you'll do almost nothing, without some copper you will absorb and use very little calcium, now calcium supplements have copper and normally potassium and some zinc in, the uptake has been increased massively. These are things people learn over time and decades ago we didn't know. Likewise most vitamins need the right balance of other things in them to make the body absorb them well.
In other words, our normal sources of vitamins are getting much worse, and our supplement versions due to more and more knowledge are getting far better. its why supplementing things like selenium, and omega's can be so important, because they were widespread 100 to 3000 years ago, and the levels have dropped dramatically in the past 100 years. you eat the same diet now as 100 years ago and you'll get almost no selenium, and far far less omega 3 and far far more omega 6, too much omega 6 and you won't absorb omega 3 that well.
its all about balance but our "natural" food's balance has been changing for decades, you need to account for this.