While researching a post on vegan sources of omega-3’s, I noticed that there is almost no information on the internet about whether flax seeds can cause of oral allergy syndrome.
Oral allergy syndrome (OAS) occurs when an allergy (typically a pollen allergy) causes an allergic response to a similar protein in a plant food. Oral allergies vary in severity, although in my case they are just a cause of irritation — itching around the mouth and throat, and indigestion. Oral allergies are typically only triggered by raw foods, as heating often denatures the allergen, resolving the allergy.
Oral allergies are often diagnosed by having reactions to well-known common oral allergy foods (such as those on this list) that is resolved by cooking the foods. I have been diagnosed with oral allergy syndrome, and I react to bananas, almonds, melons, carrots, and occasionally pears or apples — these are all common OAS foods.
However I have identical symptoms to raw flax seeds, which I’ve never seen mentioned in lists of oral allergy foods. Is this really the same thing? If so, am I the only one? I wanted to research this to see if my flax sensitivity could be an oral allergy.
I was able to identify only one study of flax seed allergic sensitivity that tried to distinguish true flax allergies from crossover oral allergy syndrome. This was a French study made up of a mix of allergic (one food or respiratory allergy) and non-allergic people.
The study found only two cases of true flax allergy in the 1317 people, compared with 75 people who were sensitive due to cross-reaction (more or less equivalent to oral allergies). Since allergic people were overrepresented in this study, the rate in the general population would be even lower. The authors project that about 1 in 6000 people (in France) are allergic to flax seeds, whereas 0.5% to 1% of the population have cross-reactive sensitivity only to raw flax seeds.
That is, not only can flax seeds cause oral allergy syndrome, but the vast majority of flax seed sensitivity is caused by OAS-like crossover allergies.
First, some caveats: Since so few cases of flax seed allergies have been studied, it’s hard to definitively say how that allergy presents. We could also expect the rate of flax allergies to vary between regions because of different levels of environmental and dietary allergens. This study was conducted in France, and I’m not aware of another study taking place elsewhere. Finally, while rare, there have been cases of life-threatening anaphylaxis caused by flax seeds, so please talk to a doctor if you might be allergic to flax seeds!
That said, this study suggests that flax seed sensitivity can be caused by oral allergy syndrome, and in fact the vast majority of people with flax seed sensitivity — 97% — have OAS-like crossover allergic reactions, rather than a true allergy.
If you react to raw flax seeds, but can eat cooked flax without irritation, you’re not crazy. You are probably among the estimated 1% of people who have an oral allergy syndrome sensitivity to uncooked flax seeds.
 Fremont S, et al. Prospective study of sensitization and food allergy to flaxseed in 1317 subjects. Eur Ann Allergy Clin Immunol. 2010 Jun;42(3):103-11. PMID: 20648772.
 Authors project a 1 in 6000 rate of flax allergies compared to a 0.5 to 1% rate of flax cross-sensitivity. Ratio = (0.5 / 100) / (1/6000) = 0.5 / (1 / 60) = 0.5 * 60 = 30. Percentage = 100 / 30 = 3.33% (96.66%). If we use the 1% rate, we get 1.66% (98.33%). 97% is a conservative intermediate.