On 23 October 2026, the Athletics Integrity Unit (AIU) provisionally suspended Peres Jepchirchir — the 2021 Olympic champion and 2025 world champion in the marathon. According to media reports, erythropoietin (EPO) was found in an out-of-competition sample. A provisional suspension is not a final decision: the case is only just beginning, no sanctions have been imposed yet, and the athlete has the right to a defence.

But the drug itself deserves a closer look. EPO is the best-known doping agent in endurance sports, and two opposing legends surround it: “the elite can’t run without it” and “it’s been detectable for ages, nobody takes the risk”. Let’s see what the research says.

What EPO is

Erythropoietin is a hormone produced by the kidneys when tissues are short of oxygen. It makes the bone marrow produce more red blood cells. More red blood cells mean more haemoglobin, which means the blood carries more oxygen. Recombinant EPO was developed to treat anaemia in kidney disease and cancer. On the WADA list it is prohibited at all times.

The natural way to raise the same hormone is altitude: in hypoxia the kidneys release more EPO on their own. That is the basis of “live high, train low” training.

A randomised trial: the lab versus the mountain

The most rigorous test was carried out by Dutch researchers Heuberger and colleagues (The Lancet Haematology, 2017). 48 well-trained amateur cyclists aged 18–50 were randomly assigned to receive either epoetin beta (on average 6000 IU per week) or placebo for 8 weeks, double-blind.

The results:

haemoglobin rose: 9.6 versus 9.0 mmol/L;
maximal power in a graded test: 351.55 versus 341.23 W — about +3%;
VO₂max: 60.1 versus 57.4 mL/min/kg — about +5%;
45-minute submaximal power test: 283.18 versus 277.28 W — difference not significant;
race to the summit of Mont Ventoux: 1:40:32 in the EPO group versus 1:40:15 on placebo — no difference.

Side effects were mild (grade 1–2) and the same in both groups.

The authors’ conclusion: EPO improved a laboratory test of maximal work, but had no effect on the more sport-like submaximal test and road race.

The study drew plenty of criticism. The participants were not professionals, and an uphill race is a single effort with many external factors. Picking out the 1–2% difference that decides the outcome at elite level is very hard in such a setting. So “EPO is useless” does not follow from this study. Something else does: a rise in VO₂max of a few percent does not automatically turn into the same gain over a race distance.

EPO at altitude didn’t help either

In 2025, the Journal of Applied Physiology published another randomised trial (Bradbury and co-authors, universities of Oregon and Colorado). 39 people (12 women), for about 3.5 weeks before a two-week stay at 3094 m, received placebo (13 people), intravenous iron (14) or EPO (12; epoetin alfa 50 IU/kg three times a week).

Haemoglobin mass at altitude rose equally in everyone. 5 km times at altitude were slower than at sea level and did not improve in any group over the 14 days. Altitude sickness did not differ either. At these doses, neither iron nor EPO gave any advantage.

Microdoses: how many days they stay visible

The classic way to get around testing is microdosing: a small dose of EPO is cleared quickly, while the effect builds up. Two recent studies checked how long it remains visible.

Karolinska Institute, Stockholm (Drug Testing and Analysis, 2025). Healthy men received a single subcutaneous microdose of EPO — 15 IU/kg. Standard WADA methods detected it in urine for up to 72 hours. The researchers also tested dried blood spots from different self-sampling devices: one of them turned out to be less sensitive than the other two.

Catalan Antidoping Laboratory, Barcelona (Journal of Pharmacological and Toxicological Methods, 2026). 4 participants, the same microdose of 15 IU/kg. A dried blood spot — just one spot of a self-sampling device, 17.5 µL — showed EPO for up to 72–96 hours, urine for up to 84–96 hours. The samples remained stable for at least 6 months at room temperature and at −20 °C. This means a drop of blood can be taken from an athlete at home and re-tested later with new methods.

Another layer of control is the biological passport: it tracks not the drug itself, but changes in blood markers over months.

What not to believe too firmly here

The randomised trials involved amateurs, not the elite. At the 2:15 marathon level, the outcome is decided by percentages that are statistically invisible in studies like these.

Doses in studies are not doses in real doping. The regimens used by cheats cannot be reproduced within the ethics of clinical trials.

The microdose studies are small. 4 people in Barcelona is a test of a method, not an estimate of how many cheats the system misses.

The Jepchirchir case is not proof of guilt. It is a provisional suspension, and it is too early to draw conclusions about the individual athlete.

What this means for amateurs

There are no legal “EPO boosters”. Supplements sold “to raise erythropoietin” have no convincing data on performance. Our look at SARMs in recreational athletes shows where chasing grey-market drugs leads — including bans at age-group races.
If your haemoglobin is low, look for the cause. The most common one in runners is iron deficiency. We wrote about how to spot it from ferritin in a separate article.
A higher VO₂max is not the same as a better result. Even an artificial gain of almost 5% did not win the race. It is useful to know your VO₂max, but over a race distance it is threshold, economy and endurance that do the work.
Altitude is the legal route to the same hormone. But there are no guarantees here either: in a 2025 study, two weeks at 3094 m did not improve 5 km running in any group. The effect depends on the protocol and duration, but at least nobody gets banned for it.

The bottom line

On 23 October 2026, Olympic and world marathon champion Peres Jepchirchir was provisionally suspended over a test for EPO; this is not a final decision.
A 2017 RCT: 8 weeks of EPO in 48 cyclists raised haemoglobin, maximal power (+3%) and VO₂max (about +5%), but did not change the 45-minute test or the climb time up Mont Ventoux.
A 2025 RCT: EPO and iron before two weeks at 3094 m did not improve haemoglobin mass or 5 km running compared with placebo.
A 15 IU/kg microdose is visible in urine for up to 72 hours (Stockholm) and up to 84–96 hours (Barcelona), and in a dried blood spot for up to 72–96 hours, and such samples keep for at least six months.

Sources: Heuberger J.A.A.C., Rotmans J.I., Gal P. et al. “Effects of erythropoietin on cycling performance of well trained cyclists: a double-blind, randomised, placebo-controlled trial”. The Lancet Haematology, 2017;4(8):e374-e386. DOI: 10.1016/S2352-3026(17)30105-9 · Bradbury K.E., Gideon E.A., Baranauskas M.N. et al. “Impact of intravenous iron or exogenous erythropoietin on hemoglobin mass, exercise performance, and acute mountain sickness during altitude acclimatization”. Journal of Applied Physiology, 2025;139(4):954-963. DOI: 10.1152/japplphysiol.00076.2025 · Heiland C., Hopcraft O., Johanson M. et al. “Detection of a Single Microdose of Testosterone and Recombinant EPO in Healthy Volunteers”. Drug Testing and Analysis, 2025;17(11):2220-2227. DOI: 10.1002/dta.3932 · Requena-Tutusaus L., Anselmo I., Coll S., Ventura R. “Detection capabilities of a single dried blood spot for recombinant erythropoietin micro-dose testing in sport”. Journal of Pharmacological and Toxicological Methods, 2026;139:108423. DOI: 10.1016/j.vascn.2026.108423 · ABC News. “Sydney marathon champion Peres Jepchirchir receives provisional doping ban over EPO positive test”, 24.10.2026