Herb-Drug Interactions: Flaxseed

2011

Linum usitatissimum L. (Linaceae)

Synonym(s) and related species

Flax, Linseed.

Constituents

The seeds contain a fixed oil, composed of glycerides of linoleic and linolenic acid. The seeds also contain: mucilage; the lignans secoisolariciresinol and its diglucoside; and the cyanogenetic glycosides linamarin and lotaustralin.

Use and indications

Flaxseed was formerly used as a demulcent and soothing emollient agent for bronchitis and coughs, and applied externally to burns. More recently, flaxseed oil has been used to lower blood-cholesterol levels, and flaxseed extract is being taken as a form of hormone replacement therapy due to its phytoestrogenic effects, thought to be due to the lignans (although note that the information available on phytoestrogenic lignans is limited).

Pharmacokinetics

Ingested lignans such as secoisolariciresinol have been shown to undergo bacterial hydrolysis and metabolism to produce the mammalian lignans enterolactone and enterodiol, which have oestrogenic effects.

Interactions overview

Flaxseed lignan supplementation appears to have no significant effect on blood-glucose levels in type 2 diabetic patients also taking oral antidiabetic drugs (not named). Limited evidence suggests that flaxseed oil may increase bleeding times and therefore some caution might therefore be appropriate with aspirin and anticoagulants.

Flaxseed + Anticoagulant or Antiplatelet drugs

Limited evidence suggests that flaxseed oil may have some antiplatelet effects, which could be additive with those of conventional antiplatelet drugs, and increase the risk of bleeding with anticoagulants.

Clinical evidence

Two case reports briefly describe increased bleeding (haematuria and nosebleeds) in patients taking aspirin and flaxseed oil, one of whom was taking low-dose aspirin.

Some studies have investigated the effect of flaxseed oil alone on bleeding time, and one, in 10 healthy subjects, found that a flaxseed oil rich diet (20.5 g daily of alpha-linolenic acid) for 56 days had no significant effect on bleeding times, prothrombin times or partial prothrombin times. However, another study in 11 patients with rheumatoid arthritis reported that flaxseed oil 30 g daily for 3 months (9.6 g daily of alpha-linolenic acid) increased the bleeding time by about one minute when compared with baseline, although this result was not statistically significant.

Experimental evidence

No relevant data found.

Mechanism

Omega-3 fatty acids such as linolenic acid are thought to have some antiplatelet effects and might therefore prolong bleeding time. Theoretically, this effect might be additive to that of other antiplatelet drugs, and increase the risk of bleeding with anticoagulants.

Importance and management

The general significance of these reports is unclear and no interaction has been established. Nevertheless, a large epidemiological study would be needed to quantify any excess risk in the order of that seen with antiplatelet doses of aspirin taken with warfarin. As with high doses of fish oils (marine omega-3 fatty acids), it may be prudent to use some caution with the concurrent use of high doses of flaxseed supplements in patients also taking aspirin or anticoagulants.

Flaxseed + Antidiabetics

Flaxseed lignan supplementation appears to have no significant effect on blood-glucose levels in type 2 diabetic patients also taking oral antidiabetic drugs.

Clinical evidence

In a randomised, crossover study in 68 patients with type 2 diabetes and mild hypercholesterolaemia, taking a supplement containing a total of 360 mg of flaxseed lignan daily for 12 weeks had no significant effect on blood-lipid profile, insulin resistance, fasting glucose and insulin concentrations. A minor reduction of glycosylated haemoglobin (HbA1c) of about 0.1% occurred, although the clinical significance of this reduction is likely to be minimal. In this particular study, patients continued to take their usual medication, which included oral antidiabetics and lipid-lowering medications, none of which was specifically named in the study. Patients were excluded from the study if they were using insulin. Similarly, in another study, flaxseed oil (60 mg/kg alpha-linolenic acid daily) had no significant effect on blood-glucose control in type 2 diabetics. Patients taking insulin were also excluded from this study; however, information on other concurrent medication was not reported. In another study in 25 menopausal women with hypercholesterolaemia, there was a slight 5.3% reduction in blood-glucose levels (0.1 mmol/L) with crushed flaxseed, and this was less than that seen with conventional HRT, which is not considered to have blood-glucose-lowering effects.

Experimental evidence

No relevant data found.

Mechanism

No mechanism expected.

Importance and management

It appears from these studies that flaxseed oil or lignans have minimal effects on glycaemic control in type 2 diabetes, and in one study the lignans had no additive blood-glucose-lowering effects with oral antidiabetic drugs (not named). Flaxseed is therefore unlikely to affect the blood-glucose-lowering efficacy of concurrent antidiabetic medication. However, more detailed information on specific antidiabetic drugs is unavailable.

Flaxseed + Food

No interactions found.

Flaxseed + Herbal medicines

No interactions found.