You can take most types of fish oil with a multivitamin, but use caution when taking cod liver oil.
Multivitamin and Fish Oil
There's no best time to take fish oil. Taking your fish oil supplement and your multivitamin together may affect absorption of certain nutrients, but it's not clear by how much. The absorption of the fat-soluble vitamins A, D, E and K may be enhanced when taken with a meal including fat. Taking those nutrients in combination with fish oil could have a similar effect.
Fish Oil Versus Liver Oil
The majority of fish-based, omega-3 supplements you'll see in stores and online are fish oils, obtained from pressing the whole body of the fish. Fatty fish including salmon, herring, mackerel, anchovies and sardines are the most common sources. Liver oils are obtained by pressing only the liver of the fish, which is usually cod.
Problems with Excess A and D
Unlike water-soluble vitamins, excess amounts of which are excreted in the urine, fat-soluble vitamins are stored in the fat cells for long periods of time. Getting too much of these nutrients can cause them to build up to dangerous levels in the body, leading to side effects and more serious health consequences.
Hypervitaminosis A is an especially concerning condition, in which chronic high intakes can cause liver damage, according to the NIH. Other side effects may also include headache, diarrhea, nausea, dizziness and pain in the joints and bones. The tolerable upper intake level, called UL, for adults for vitamin A is 3,000 micrograms or 10,000 international units. Exceeding this amount on a regular basis increases your risk of liver toxicity and other negative side effects.
Should You Take Both?
In conservative doses, there's no reason not to take a multivitamin and fish oil, especially if your diet is not as healthy as it could be and/or you don't eat seafood. If you take fish oil, and not cod liver oil, you don't have to worry.
However, you should be sure to read the labels of your chosen fish oil and multivitamin products to make sure that, combined, you are not exceeding safe intake levels. You should also double check the labels of other supplements you take, because they may also contain additional nutrients such as vitamins A and D.
If you eat a healthy diet, including fresh fish twice weekly, chances are that you don't need a fish oil supplement, according to the Harvard T.H. Chan School of Public Health. The same is true for multivitamins.
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