Flaxseed is the seed of the flax plant. Flaxseed oil (also called linseed oil) comes from flaxseeds and is available in liquid and capsule form. Whole, crushed or powdered flaxseed can be mixed with water or juice and taken orally, or can be sprinkled on foods. Although flaxseed contains many healthy components, it is primarily made up of three ingredients: Omega-3 essential fatty acids ("good" fats that have been shown to have heart-healthy effects), lignans (which have both plant estrogen and antioxidant qualities) and fiber (both the soluble and insoluble types).

Since flaxseed contains soluble fiber, like that found in oat bran, it is considered an effective laxative. Studies of flaxseed have been shown to lower cholesterol levels, also thought to be a benefit, report mixed results. Additionally, some studies suggest that alpha-linolenic acid, also called ALA (a substance found in flaxseed and flaxseed oil) may benefit people with heart disease. But not enough reliable data is available to determine whether flaxseed is effective for heart conditions.

There are some differences between flaxseed and flaxseed oil that should be noted. Flaxseed contains lignans (phytoestrogens, or plant estrogens), while flaxseed oil preparations lack lignans; flaxseed oil alone contains neither the fiber nor the phytochemicals of whole flaxseed.

Flaxseeds need to be ground to make the nutrients available, otherwise they just pass through. Flaxseed and flaxseed oil supplements seem to be well-tolerated and few side effects have been reported. However, since flax has such a high fiber content, it's best to start with a small amount and increase your intake slowly, otherwise cramping and a "laxative effect" can result. People with Irritable Bowel Syndrome may have an especially strong reaction to it. Flaxseed, like any supplemental fiber source, should be taken with plenty of water; otherwise, it could worsen constipation or, in rare cases, even cause intestinal blockage. The fiber in flaxseed may also lower the body's ability to absorb oral medications, so it's often advised that flaxseed should not be taken at the same time as any conventional oral medications or other dietary supplements. The optimum dose to obtain health benefits is not yet known, but one to two tablespoons of ground flaxseed a day is currently the suggested dose, according to the Flax Council of Canada.