Liane Llewellyn Hickling, 34, from Bradford, was pulled out of the water in the Menai Strait on Wednesday night.
She had suffered an allergic reaction after being stung by at least six Lion's Mane jellyfish.
She was treated at Ysbyty Gwynedd, Bangor, and is now recovering at her base in Holyhead.
Mrs Llewellyn Hickling had set off from Moelfre on Wednesday morning and had hoped to complete the 70 mile (112km) course in less than 48 hours.
Following British Long Distance Swimming Association rules, she was wearing just a swimsuit, swimming hat and goggles.
Her husband Chris Hickling said: "She was stung two or three times as she crossed Red Wharf Bay and again when she got to Penmon. She suffered more stings in the Menai Strait and that's when we decided to pull her out.
"The stings can be really nasty and we think she's had a bad allergic reaction. It's going to take her a few days to recover fully.
"It's too early to say whether she'll attempt the swim again."
Mrs Llewellyn Hickling, an NHS physiotherapist, had been raising money for a new RNLI lifeboat to be stationed at Barmouth.
It will be named in honour of crewman Craig Steadman, who died in a motorbike accident last year.
"She really wants people to continue to donate, even though the swim had to be called off," added Mr Hickling.
As one of the UK's most-experienced long distance open water swimmers, Mrs Llewellyn Hickling has swum the length of Windermere, Loch Lomond, Loch Ness and, in 2009, became only the third British woman to swim the English Channel, there and back.
As part of her preparation for the rough waters and tides of Anglesey, she completed the first recorded swim around St Kilda in Scotland's Outer Hebrides in June 2016.
A woman's attempt to become the first person to swim non-stop around Anglesey has been called off after she was stung by jellyfish.