Olive Oil Side Effects: What the Research Actually Shows

By Nicolas Netien, Chief Scientist at Oleaphen, who holds the world record for the highest polyphenol concentration ever measured in olive oil (2018).

People ask me about side effects more than almost anything else, and I understand why. The oil I make sits at the far end of the concentration scale, and when a number climbs that high, the honest first question is whether it is safe to take every day. So here is what the clinical record actually says, kept separate from what the marketing and the worry both tend to add on top.

TL;DR

In clinical studies, high polyphenol olive oil has been well tolerated across a wide range of daily doses. The effect reported most often is mild, short-lived digestive upset such as loose stool, and it shows up mainly when intake goes well past normal food amounts. It settles quickly once you ease back. Allergy to ingested olive oil is very rarely documented. If you take blood-thinning medication or live with a serious digestive condition, check with your doctor before adding a concentrated daily dose. The EU authorised intake for the heart-health claim is 20g a day containing at least 5mg of hydroxytyrosol and its derivatives, for the protection of blood lipids from oxidative stress.

Quick-reference table

Concern	What the research shows
Digestive upset	The main effect reported in trials is mild, temporary loose stool, mostly at intakes well above normal food amounts. It resolves when intake drops.
Drinking it on an empty stomach	Larger single amounts taken on an empty stomach are the most common trigger for that mild looseness. Splitting the dose or taking it with food reduces it.
Allergic reaction	Allergy to eaten olive oil is very rarely documented in the scientific literature.
Blood-thinning medication	People taking anticoagulants should check with a doctor before a concentrated daily dose, because olive oil carries vitamin E.
Serious digestive conditions	Anyone with a significant gastrointestinal condition should speak to their clinician before adding a daily dose.

How olive oil polyphenols behave in the body

Olive oil carries a set of phenolic compounds you do not find together anywhere else, mainly oleocanthal, oleuropein and hydroxytyrosol. They arrive dissolved in the oil's own fat, which is part of why they are absorbed well and why olive oil has been studied so closely on its own terms rather than lumped in with polyphenols from tea or fruit.

The effects are dose-dependent, and that cuts both ways. Higher phenolic concentrations have been linked in trials to clearer cardiovascular and anti-inflammatory readings, and a higher dose is also where the one documented physical effect, mild digestive looseness, starts to appear. Neither side of that is dramatic at the amounts people actually consume.

The one effect that shows up in studies: mild digestive upset

Across the clinical literature, the side effect reported with any regularity is gastrointestinal. Taking a large amount of olive oil at once, especially on an empty stomach, can cause temporary loose stool or mild diarrhoea. It is the kind of effect that arrives when intake goes well beyond a normal food portion, and it clears quickly once intake drops.

This is why the timing questions come up so often, whether olive oil is harder on the stomach in the morning, or taken before bed. The pattern in practice is simpler than the timing: it is the size of a single dose on an empty stomach that matters more than the hour. Splitting the amount across the day, or taking it with food, makes the mild looseness far less likely while keeping the benefits that the trials describe.

There is a flip side worth stating plainly. Research has also linked regular, moderate olive oil intake with a lower reported incidence of inflammatory bowel conditions and irritable bowel syndrome. So the same food that can loosen the gut at an excessive single dose is associated with better digestive outcomes at sensible daily amounts.

What happens at very high concentrations

Most people taking concentrated, polyphenol-rich olive oil do so in small daily amounts, around 5 to 10ml, rather than the larger culinary pours used in cooking. That small format is the point: you get the phenolic dose without the calorie load of a tablespoon or more. Some producers, ours included, work above 2000 mg/kg and supply the oil in small sealed doses for exactly this reason.

One rat study is worth addressing directly, because it is the single piece of research that looks unfavourable at first glance. Veskoukis and colleagues fed rats a high-polyphenol olive oil (around 800 mg/kg) and found it strengthened the antioxidant profile of blood, brain, muscle and the small intestine, while appearing to induce oxidative stress in the spleen, pancreas, liver and heart. The authors' own reading is the part that usually gets dropped: they interpret that tissue-specific response not as damage but as an adaptive adjustment under long-term, low-dose exposure, the kind of hormetic response now well described in toxicology. It is one animal study, and the researchers themselves frame the effect as an adaptation rather than harm.

Allergy to olive oil is rare

Allergic reactions to olive oil applied to the skin are uncommon, and reactions from eating it are close to absent in the published literature. That makes high polyphenol olive oil suitable for most people, including many who react to other foods. A genuine olive allergy exists but is rare, and is the one clear case where the oil should be avoided.

Who should check with a doctor first

For the large majority of people, daily olive oil needs no special clearance. A few situations are worth a conversation with a clinician before starting a concentrated daily dose:

• People on blood-thinning medication, because olive oil contains vitamin E, which can interact with anticoagulant therapy.

• People with a serious digestive disorder, where any new daily fat intake is worth discussing first.

• People with a known olive allergy, which is rare but real.

None of these is a reason for alarm. They are the ordinary cases where personalised medical advice beats a general guide.

How much is safe to take

The clearest anchor is the European Food Safety Authority's review. The EU authorised health claim is built on 20g of olive oil a day, containing at least 5mg of hydroxytyrosol and its derivatives, for the protection of blood lipids from oxidative stress under Regulation 432/2012. That figure comes after a formal safety evaluation of the available evidence.

In trials, daily amounts have ranged widely, from a few millilitres up to 40 to 70 grams, without reported adverse effects. For concentrated, high-phenolic oils, a teaspoon-sized daily amount delivers a meaningful phenolic dose while staying comfortably inside that tested range. Total daily polyphenol intake from a full diet sits somewhere between 0.1 and 1.0 grams, drawn mostly from fruit, vegetables, coffee, tea and wine, so even a concentrated oil contributes only a portion of the day's total.

A practical note on quality, since it bears on safety as much as on potency: choose oil from producers who publish third-party testing by accredited methods such as LC-MS/MS or HPLC, and who screen for pesticide residue and contaminants. The phenolic number on a label is only as trustworthy as the method behind it.

Common questions

Does olive oil have side effects?

For almost everyone, olive oil is well tolerated. The only effect reported with any regularity in clinical studies is mild, short-lived digestive upset, and it appears mainly when intake goes well past normal food amounts.

Can high polyphenol olive oil upset your stomach?

A large amount taken at once, especially on an empty stomach, can cause temporary loose stool. Splitting the dose or taking it with food makes this far less likely.

What are the side effects of drinking olive oil?

The main one is mild, temporary diarrhoea at excessive single doses. It resolves quickly once intake is reduced.

Is it safe to take olive oil every day?

Yes for most people. The EU authorised intake for the heart-health claim is 20g a day containing at least 5mg of hydroxytyrosol and its derivatives, and trials have used daily amounts well above that without reported adverse effects.

Are people allergic to olive oil?

A true olive allergy exists but is rare, and reactions from eating olive oil are close to absent in the published literature.

The bottom line on Olive Oil side effects

The weight of the evidence points one way. High polyphenol olive oil has a strong safety record across population studies and controlled trials.

1. No serious adverse effects have been reported in the clinical research on olive oil polyphenols.

2. The European Food Safety Authority reviewed the safety evidence and authorised a daily heart-health intake.

3. The only commonly documented physical effect is mild digestive looseness at excessive single doses, which clears quickly.

4. Allergy from eating olive oil is very rarely documented.

5. A few groups, those on blood thinners, those with serious digestive conditions, and those with an olive allergy, should check with a doctor first.

For most people the more useful question is not whether olive oil is safe at sensible amounts, which the record supports, but whether they are getting enough of these compounds in the first place.

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Scientific references

1. Covas, M.I., et al. (2006). The effect of polyphenols in olive oil on heart disease risk factors: a randomized trial. Annals of Internal Medicine, 145(5), 333-341.

2. European Food Safety Authority. (2011). Scientific Opinion on the substantiation of health claims related to polyphenols in olive oil. EFSA Journal, 9(4), 2033.

3. Fitó, M., et al. (2007). Anti-inflammatory effect of virgin olive oil in stable coronary disease patients: a randomized, crossover, controlled trial. European Journal of Clinical Nutrition, 62(4), 570-574.

4. Konstantinidou, V., et al. (2010). In vivo nutrigenomic effects of virgin olive oil polyphenols within the frame of the Mediterranean diet: a randomized controlled trial. FASEB Journal, 24(7), 2546-2557.

5. Martín-Peláez, S., et al. (2013). Health effects of olive oil polyphenols: recent advances and possibilities for the use of health claims. Molecular Nutrition & Food Research, 57(5), 760-771.

6. Veskoukis, A.S., et al. (2020). Olive oil with high polyphenolic content induces both beneficial and harmful alterations on rat redox status depending on the tissue. Toxicology Reports, 7, 421-432.

7. Sarapis, K., et al. (2020). The effect of high polyphenol extra virgin olive oil on blood pressure and arterial stiffness in healthy Australian adults: a randomized, controlled, cross-over study. Nutrients, 12(8), 2272.

8. Visioli, F., & Galli, C. (2002). Biological properties of olive oil phytochemicals. Critical Reviews in Food Science and Nutrition, 42(3), 209-221.

9. Schwingshackl, L., et al. (2014). Olive oil in the prevention and management of type 2 diabetes mellitus: a systematic review and meta-analysis. Nutrition & Diabetes, 4(8), e122.

By Nicolas Netien, Chief Scientist at Oleaphen. He has spent fifteen years on the science of high phenolic olive oil, from soil biology and cultivation through to extraction, and has advised producers across the industry. He is a Knight of the Order of Agricultural Merit of the French Republic and holds the world record for the highest polyphenol concentration ever measured in olive oil, 4,943 mg/kg, set in 2018.