Cancer Research UK is to run its Dryathlon mass participation fundraising event next month - directly competing with Macmillan's annual sobriety fundraiser, Go Sober for October.
The two large cancer charities both started similar mass participation events asking fundraisers to obtain sponsorship for giving up alcohol for a month, five years ago.
CRUK launched Dryathlon in January 2013 and was followed by Macmillan, which launched Go Sober for October.
This year CRUK is encouraging its supporters to give up alcohol in October as well as January and described the move as an evolution.
Promotional material on its website encourages supporters to "take on the ultimate test of willpower by going booze-free this October" and there is a countdown calendar for participants.
Dryathlon raised £4.2m in its first year and £5m the following year. But by 2015 was down to £3.7m and £2.8m in 2016. It has not announced the fundraising total 2017, and did not appear in an annual list of the largest mass fundraising events.
In contrast Macmillan's event has continued to grow each year, and last year raised over £5m.
In 2015 CRUK began offering a second round of Dryathlon in September - urging people to give up the booze after indulging over the summer.
Over nine campaigns CRUK has raised £22m, but has declined to reveal exact figures for its most recent campaign.
Macmillan has raised nearly £20m from five campaigns.
CRUK told Civil Society News that it still plans to run Dryathlon in January 2019 and that it is considering keeping the campaign open all year.
Sarah Pickersgill, head of fundraising products at Cancer Research UK said: "Dryathlon has been successful for Cancer Research UK, raising over £22m since launching in 2013. So that Dryathlon can keep raising money for our work to beat cancer, it's essential we look at ways to evolve the campaign.
"Since 2015 we have offered people the flexibility to take part in Dryathlon in January or September. This year we are trialling Dryathlon in October as an initial step to keeping the campaign open all year round so that people can give up alcohol at any point to raise money for Cancer Research UK. This is something we will be looking to do across all our fundraising campaigns."