The Lost actress revealed in May she had sustained a traumatic brain injury (TBI) after fainting and falling "face-first into a boulder" and she recently underwent tests to examine how she has been impacted, but the results weren't good news.
In a video shared to Instagram, she said: "It's late on January 1, the first day of 2026, and I'm entering into this new year — the year of the horse — with some bad news about my concussion.
"The results came back from the scans, and I missed every area, and my brain is functioning at a decreased capacity.
"So, I do have brain damage from the TBI, and possibly other factors going on."
The 46-year-old actress is ready to embark on the "hard work" of "fixing" her medical problems and noted the positives of her health struggles meant she had enjoyed a "restful" end to the year.
She continued: "Now, my job is to get to the bottom of that with the doctors, and then embark on the hard work of fixing it.
"I don't look forward to it because I feel, like, hard work is all I do. That's OK, my cognitive decline since I smashed my face open has helped me to slow down and helped me to have a more restful finished 2025."
Noting she'd experienced "the most restful holiday" season she'd ever had, Lilly added: "That's a good thing. (I'm) feeling extraordinarily grateful and blessed to be able to play one more day, one more year on this beautiful living planet. That is my concussion update. Thank you all for caring."
The Ant-Man actress has an "uphill battle" to get back to full health.
She captioned her video: "Comforting to know my cognitive decline isn't just peri-menopause, discomforting to know what an uphill battle it will be to try to reverse the deficiencies.
"Thank you all for always asking, for always caring, and for your continued prayers."
Lilly detailed her accident on Instagram in May.
She wrote on Instagram at the time: "I fainted at the beach, and fell face first into a boulder.
"At the hospital, the nurses and doctor went straight into action, more determined to find the cause of my blackout than to stitch up the hole punctured into my face by the rock. I smiled wryly at them. 'You won't find anything.' I said with a woozy voice."
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