Here in Australia, it’s winter. It’s currently warmer in the fridge than it is outside. We’ve just been blasted by a wall of frigid air straight from Antarctica, and much of the south-eastern corner of our continent has snow drifts over parts that not so long ago were baking under the hot Autumn sun. It’s not something we’re used to in Australia.
Of course, now that winter is firmly entrenched, more people are coming to see me with their viral upper respiratory tract infections, better known as ‘colds’. Yes, ’tis the season to be sneezin’! Over my years of practice, I’ve seen enough people with a cold to last me a thousand winters.
What always fascinates me are the things that people try to use to cure their cold. I think I’ve heard everything over the last decade: garlic, ginger, peppermint, chicken soup, honey, tea, honey mixed with lemon mixed with tea, or honey mixed with lemon mixed cayenne pepper mixed with tea. Some people rub Vicks on their feet. Other people douse their pillows in eucalyptus oil.
Another common recommendation that gets around the grape vine and social media is olive leaf extract, used in traditional ‘medicine’ for thousands of years, and those witch-doctors and shamans can’t all be wrong.
One published review described the ‘science’ of olive leaf extract: “Constituents of the olive tree, Olea europaea, have been studied and utilized in folk medicine for centuries. Olive leaf extract, derived from the leaves of the olive tree, contains phenolic compounds, specifically oleuropein, that have demonstrated potent antimicrobial, antioxidant, and anti-inflammatory activity. Oleuropein and derivatives such as elenolic acid have been shown to be effective in in vitro and animal studies against numerous microorganisms, including retroviruses, coxsackie viruses, influenza, and parainfluenza as well as some bacteria. Research suggests that olive leaf constituents interact with the protein of virus particles and reduce the infectivity and inhibit replication of viruses known to cause colds, influenza, and lower respiratory infection. Olive leaf extract has also been shown to stimulate phagocytosis, thereby enhancing the immune response to viral infection. Anecdotal reports indicate olive leaf extract taken at the onset of cold or flu symptoms prevents or shortens the duration of the disease. For viral sore throats, gargling with olive leaf tea may alleviate symptoms, possibly by decreasing inflammation and viral infectivity.” 
It’s always a concern when a supposedly peer reviewed journal allows an article to get through which seriously discusses anecdotal evidence as something worthy of attention. Anecdotal evidence is the weakest level of evidence possible. Anecdotal evidence is essentially just stories and opinion . There’s anecdotal evidence for the Tooth Fairy. The other ‘evidence’ that this review describes is from in vitro studies, which are trials in test tubes not in people. In vitro evidence is only helpful in a general sense. Just because a reaction happens in a test tube or petri dish doesn’t mean that it will happen in a real life human being.
So then, do the claims for olive leaf extract stand up to the rigors of modern scientific enquiry or is it like every other cold and flu ‘remedy’ – just another individualised mythology?
Being sceptical, I wanted to find out. So I searched through the published medical literature for quality clinical trials that studied olive leaf extract in humans, and I found only six trials. Interestingly, all of the trials studying olive leaf extract weren’t looking at its effect on immune function but on cholesterol and blood sugar control, blood pressure, and oxidative stress.
In 2009, Kendall et al published a single-centre, randomized, single-blinded, prospective pilot comparison of the effect of dietary supplementation with olive leaf extract on the markers of oxidative stress in 45 healthy young adult volunteers. They found that olive leaf extract had no effect on oxidative stress compared to the control group .
Susalit et al (2011) published a double-blind, randomized, parallel and active-controlled clinical study looking at the tolerability, cholesterol-lowering and anti-hypertensive effect of Olive leaf extract in comparison with Captopril (a common blood pressure medication) in patients with early hypertension. After 8 weeks of treatment, there were similar reductions in blood pressure in both the olive leaf extract and the blood pressure pill groups. There was a significant drop in triglyceride levels in the olive leaf extract group, but not in Captopril group .
Wainstein et al (2012) performed a randomized controlled trial on 79 adults with non-insulin dependent diabetes, comparing a single 500mg dose of olive leaf extract with placebo over 14 weeks. They measured the HbA1c (a surrogate measurement of the average blood sugar over a three month period) and plasma insulin levels. They also did studies in rats to study the mechanism of action of the olive leaf extract. In the human trials, the subjects treated with olive leaf extract exhibited significantly lower HbA1c and fasting plasma insulin levels. This effect was thought to be reflected in the rat study which suggested that olive leaf extract reduced the digestion and absorption of starch from the intestines .
de Bock et al (2013) did a randomized, double-blinded, placebo-controlled, crossover trial on 46 patients in New Zealand, over a 30 week period. The participants were middle aged and overweight. The researchers were primarily studying insulin sensitivity but they also looked at glucose and insulin profiles, cytokines, lipid profile, body composition, 24-hour ambulatory blood pressure, and carotid intima-media thickness. The olive leaf extract group had a statistically significant improvement in insulin sensitivity and responsiveness of the pancreatic β-cell. Interestingly, the olive leaf extract supplementation improved some inflammatory markers, but not others, and made no difference to the patients lipid profile, blood pressure, body composition (their body fatness), carotid intima-media thickness (a risk predictor of cardiovascular disease), or liver function .
For completeness, de Bock lead another trial, also published in 2013, although this trial was more a study of the absorption of the compounds in olive leaf extract than a study of their effects . There was a 1996 Belgian study which was written in French. I’m not very good with French, but according to the English abstract, there was no difference between the olive leaf extract and placebo in terms of blood pressure and blood sugar levels .
Reconciling the research on olive leaf extract makes for an interesting narrative. There are a couple of really strong, methodologically robust trials on olive leaf extract, and with positive results in favour of it. However, I can count them on one hand, and while the results are encouraging for proponents of olive leaf extract, there needs to be a lot more research before those claims can be made with certainty. And in contrast to its usual selling points, those positive effects for olive leaf extract were for blood sugar control, not the prevention or treatment of viral illnesses.
The bottom line – olive leaf extract may one day prove to be a useful herbal supplement, but there’s not enough clinical evidence to support it at the present moment. And there’s certainly no evidence that olive leaf extract will do anything for your viral upper respiratory tract infections.
So next time you get a cold, don’t bother spending money on olive leaf extract. Have a couple of paracetamol, a long hot shower and a good rest.
And if symptoms persist, don’t forget to see your GP.
 Roxas M, Jurenka J. Colds and influenza: a review of diagnosis and conventional, botanical, and nutritional considerations. Alternative medicine review : a journal of clinical therapeutic 2007 Mar;12(1):25-48.
 Fowler G. Evidence-based practice: Tools and techniques. Systems, settings, people: Workforce development challenges for the alcohol and other drugs field 2001:93-107.
 Kendall M, Batterham M, Obied H, Prenzler PD, Ryan D, Robards K. Zero effect of multiple dosage of olive leaf supplements on urinary biomarkers of oxidative stress in healthy humans. Nutrition 2009 Mar;25(3):270-80.
 Susalit E, Agus N, Effendi I, et al. Olive (Olea europaea) leaf extract effective in patients with stage-1 hypertension: comparison with Captopril. Phytomedicine : international journal of phytotherapy and phytopharmacology 2011 Feb 15;18(4):251-8.
 Wainstein J, Ganz T, Boaz M, et al. Olive leaf extract as a hypoglycemic agent in both human diabetic subjects and in rats. Journal of medicinal food 2012 Jul;15(7):605-10.
 de Bock M, Derraik JG, Brennan CM, et al. Olive (Olea europaea L.) leaf polyphenols improve insulin sensitivity in middle-aged overweight men: a randomized, placebo-controlled, crossover trial. PloS one 2013;8(3):e57622.
 de Bock M, Thorstensen EB, Derraik JG, Henderson HV, Hofman PL, Cutfield WS. Human absorption and metabolism of oleuropein and hydroxytyrosol ingested as olive (Olea europaea L.) leaf extract. Molecular nutrition & food research 2013 Nov;57(11):2079-85.
 Cherif S, Rahal N, Haouala M, et al. [A clinical trial of a titrated Olea extract in the treatment of essential arterial hypertension]. Journal de pharmacie de Belgique 1996 Mar-Apr;51(2):69-71.
Yes, go to your GP. Let them give you PPIs for GERD that you don’t have and steroid inhalers for asthma that you don’t have and then get chronic fatigue syndrome because you have a non-cytolytic enterovirus infection… yes go to your GP, you do that. As for me I’ll take my chances with experimentation. If you think that peer-reviewed science is all there is to know, you are either extremely ignorant or getting paychecks from big pharma.
Hi Bob, thanks for your comment. There’s certainly some ignorance on display here, but it’s not from me. If you want to experiment with your health, be my guest. Peer-review is not perfect, but it’s a darn sight better than stupidity and guesswork. All the best to you.
Well, I can understand how that poster feels about the medical community. I do understand we need more research, and I respect that. Many years to deal with my compromised immune system connected to my intestines, I was drinking strong olive leaf tea, and it helped me. I didn’t know what I had though I went to doctors for a stool test, allergy test. Years and years later, I have been recently diagnosed with IBS, but given no solution and one doctor didn’t even care. Another one who also said IBS or discussed it cared a bit, but he didn’t have much advice. Anyway, I would say olive tea can give you a serious immune system boost. At least, that has been my experience. I know how my body reacted.
Thanks for sharing your experience Basil. I appreciate your observation, though without stronger evidence of Olive Leaf Extract’s benefits, I still won’t be recommending it.
I have been using Olive Leaf Extract capsules off and on for over sixteen years with much success to fight against the cold and flu. I can count on one hand the number of times I have been sick. I even used it in a liquid form to treat gingivitis. My periodontist was very impressed. I have also read on-line by an MD that there is no natural cure for a foot fungus. I treated mine by mixing equal amounts of apple cider vinegar with water and soaked the toe for 30 minutes a day. In addition to that I painted the infected nail with tea tree oil three times a day. Toe fungus gone! Another MD impressed and became a believer.
I am truly amazed at the number of conventional doctors that have become naturopathic doctors and not the other way around.
Thanks for sharing your story. I’m glad you feel that OLE has helped you, but a single positive anecdote is not proof for the effectiveness of OLE.
Science trumps stories. It’s easy to be blinded by our cognitive biases. You might be amazed at all the “conventional doctors that have become naturopathic doctors”, but that’s not the reality, just like you might feel like OLE helps you fight off colds and flus. Again, that’s not the reality.
So by all means, you’re welcome to keep using it, but I won’t be recommending it for my patients given the lack of actual scientific evidence.
All the best to you.
Going to my GP never got me anything but scripts that made me unbelievably sick with horrible side-effects. On the other hand, taking Olive Leaf Extract has saved, healed and balanced on many, many occasions. Of course, there is no good research on it because Big Pharma cannot patent it and will not fund the research for it. It has been used medicinally for millenniums for good reason. Don’t put people off a magnificent, healing gift from God.
You’re welcome to your opinion of course, but big Pharma is nothing compared to the greedy multinational corporates that manufacture complementary and alternative products, who do not have to do clinical trials to establish the safety and efficacy of their products, or to do any research for that matter … except for market research so they can sell their products at a huge premium to the naive and gullible. So if you want to take OLE, then be my guest. I’m going to stick with the science, not the marketing.
All the best to you.