Participants claimed there was a lack of water along the course on the hottest day of the year so far, as temperatures hit 21 degrees. Marathon organisers, however, deny this.
Around 12,000 runners finished the 26.2-mile race yesterday, as 150,000 spectators lined the streets to cheer them on.
Jennifer Hogwood, who completed the marathon in five hours 44 minutes, said: “There was no water available at mile 11 or at mile 20 by the power station.
“There was a young girl being violently sick and no one even offered her any water.”
She said as the course passed through residential areas, runners were relying on the kindness of locals to cool down and stay hydrated.
Ms Hogwood said: “There was a great atmosphere in the residential areas - residents were handing out bottled water and fruit and spraying water on us from their garden hoses.
“It was a very, very hot day.”
According to social media reports, some water stations were still stocked with water but ran out of cups, leaving some runners trying to drink from their cupped hands.
Facebook user Matt Trotman heavily criticised the event, writing: “Even by the first water station there was no cups so no water.
“By most of the other water stations there was no water and in some places they’d just cleared the water station away.”
Mr Trotman claimed the water shortage continued even at the end of the course.
“No water at the finish! I mean seriously how hard is it to get that part right and just put a bottle in the finishers bag or something,” he posted.
A Brighton Marathon statement said: “Weather forecasts from the Met Office for Marathon day (April 9) never exceeded 17C in the days preceding, with 14C being the highest temperature predicted on the morning of the day itself.
“However, we decided on Saturday to implement our contingency plan for water supplies as a precaution and significantly increased them course-wide.
“The temperature of 21C that was reported at midday on race day by the Met Office, was not predicted but we were able to meet demand for water at key locations and medical care at all times.
“44,500 litres of water were made available on Race Day (an equivalent to: 3.5litres per runner) yet due to the unexpected increase in temperature at midday, many runners also used water as a cooling measure as well as to drink.
“Supplies became depleted in one particular area of the course early into the race but at no point was there no water available at the event. When supplies were recorded to be low extra fluid was purchased and sent to the latter stages of the Brighton Marathon. Standpipes providing a fresh water supply were also used in the secondary finish and on the beach.
“Care of the runners is our primary concern and was under constant review throughout the day with appropriate measures taken wherever necessary. We have superb medical facilities and teams in place across the course, that are equipped and prepared to deal with issues such as warmer conditions, including recommending ongoing care elsewhere if necessary.
“A full debrief of any issues will take place for the development of plans should similar unexepected weather conditions arise in future years.”