Dozens of people who took part in an "extreme" obstacle race say they suffered bouts of vomiting and diarrhoea following the event.
About 3,000 took part in the X-runner mud run, which involved open water swimming, at the National Water Sports Centre in Nottingham on Saturday.
One runner said she believed "poor quality water" had caused her illness.
X-runner said it was sorry to hear competitors had fallen ill but water test results were all "normal".
Samantha, who did not want to give her full name, said she started being sick on Monday morning and was forced to take days off work.
She suffered "horrendous" stomach cramps, back pain, headaches and fever, along with her friends who also took part.
After reading X-runner's Facebook page, she realised other people had also been taken ill in the days after the event.
Another competitor named Sarah said she and most of her friends who took part have been suffering from vomiting and diarrhoea.
She blamed the "water quality" for her illness and said she was "disappointed" with the organisers' response when she contacted them following medical advice.
"X-runner are telling us it is rare to be very poorly and only a few people are ill," she said.
"This is incorrect. Nine out of 11 of our team are suffering the same or similar symptoms."
She believes heavy rain on the morning of the race could have affected the water.
Another competitor, Jane Renwick, from Derbyshire, said five of the seven in her team were struck down.
She was unable to keep any food down for five days, only managing two biscuits on Friday morning.
"I've been so ill it's indescribable, just horrific. I've been to the doctors twice and I'm due to go again to have my blood taken for testing," she said.
"I came down with stomach cramps on Sunday evening and then in the early hours of Monday I was violently ill. I've never been so ill in all my life - and I'm fit."
She added it felt as if "someone didn't do their job properly" and would be speaking to the organisers.
An X-runner spokesman said about 25 people contacted them after experiencing "a mild stomach upset".
He said this can occur when swimming in open water and had advised competitors of the risk beforehand.
The statement added: "We are sorry to hear that some people have been poorly, especially as [water] test results came back within the normal operating parameters, and recommend that any participant experiencing similar symptoms consult their GP."
Brian Taylor, chair of the trust that runs the National Water Sports Centre, said: "Our facilities use water-supplied from a variety of natural sources including surface streams and the River Trent and are similar to those found in UK coastal sea waters.
"These waters naturally have fluctuating levels of bacteria and are therefore tested regularly to safeguard the health of our customers. The results of these tests prior to the event were within normal parameters."
What did entrants have to do?
- X-runner describes their event as one of the "toughest obstacle mud runs in the UK"
- It involved 60 obstacles covering tough terrain and open water over a distance of 5k or 10k
- Competitors also had to swim, with buoyancy aids, in the rapids section of a white water course
- X-runner said all participants signed a disclaimer as part of the sign-up process and potential risks around water quality were outlined
- It said the vast majority of people taking part have not been unwell