"I'm back at square one where I don't know what's going on with me," the actor said on the latest edition of her podcast MeSsy, along with co-host Jamie-Lynn Sigler, who has been public about her own battle with MS.
Applegate first publicly announced in 2021 that she had been diagnosed with the disorder that causes the immune system to attack the cells in the central nervous system.
On Monday's podcast, Applegate revealed that despite the misdiagnosis, she still has "every symptom of someone with MS," adding "I still have to use wheelchairs when I go to the doctor's office. I can't walk from the car to the room".
She said: "It's hard to say that (my doctor) came into my room and she said, 'Congratulations, you don't have MS.' I was not happy at all.
"You don't get misdiagnosed and then all of a sudden I'm better. I haven't gotten better. If anything, I've gotten worse in some ways."
There is no single test for MS, and a diagnosis usually involves ruling out other causes.
It can take years and seeing multiple doctors before one puts the clues together.
Applegate said the news had put her in an odd place since she and Sigler have become prominent advocates for people with MS.
"It's weird because I feel like I let everybody down," she said.
"It's very unnerving, and it's really confusing for me."
Most autoimmune diseases, especially in adults, are not caused by a specific gene defect.
Instead, a variety of genes that affect immune functions can make people susceptible.
Scientists say it then takes an "environmental" trigger, such as an infection, smoking or pollutants, to set the disease in motion.
On the podcast and earlier this year while promoting her memoir You with the Sad Eyes, the Emmy-winning Applegate has been candid about unpredictable symptoms that have forced her to use diapers and scoot down stairs on her bottom.
"My life has in some ways shrunk to the size of my California king (mattress)," she writes in her book.
with PA
