Hormone replacement therapy might protect a woman’s brain against Alzheimer’s disease, but only if the timing’s right, researchers say.

Women who start hormone therapy within five years of menopause can lower their Alzheimer’s risk by as much as 32%, researchers reported this week at the annual meeting of the American Neurological Association in Baltimore.

But waiting to start hormone therapy until 65 or later increases a woman’s risk of Alzheimer’s, researchers found.

“Starting hormone replacement therapy early may give the brain some protection, but if a woman already has Alzheimer’s or memory problems, hormone therapy won’t slow them down,” lead researcher Dr. FNU Vaibhav, a student at the Pandit Bhagwat Dayal Sharma Post Graduate Institute of Medical Sciences in Rohtak, India, said in a news release.

“It’s like watering a plant: it helps when the plant is growing, but if it’s already wilting, it might be too late,” Vaibhav added.

Menopause causes a woman’s natural estrogen levels to drop, researchers said in background notes.

Hormone replacement therapy can ease symptoms caused by this estrogen decline like hot flashes, night sweats and poor sleep, researchers said. Studies also have shown hormone therapy can help with heart health and age-related bone loss.

Just under 5% of American women now use hormone therapy, down from nearly 26% in 1999, researchers said.

In a first-of-its-kind analysis, researchers pooled data from more than 50 previous studies comparing women who used hormone replacement therapy to those who didn’t.

Results showed that women who started hormone therapy around the time of menopause reduced their Alzheimer’s risk by 22% to 32%.

However, those who started hormone therapy at age 65 or later, long after menopause, had a 38% increased risk of Alzheimer’s, particularly in women whose therapy included the hormone drug progestin.

Researchers speculated that hormone therapy around the time of menopause might protect brain health by improving communication between brain cells and reducing inflammation.

However, using hormone therapy once the brain has developed early signs of Alzheimer’s might spur on the disease by triggering inflammation or stressing the brain’s blood vessels, researchers said.

“The evidence isn’t strong enough to suggest hormone replacement therapy should be taken to prevent Alzheimer’s,” Vaibhav said. “However, if a woman is planning to use it for menopause symptoms, starting soon after menopause might give her brain some protection against Alzheimer’s disease later.”

Vaibhav said a woman should talk to her doctor about stopping it after a few years to avoid increasing her risk.

“And women should not start hormone replacement therapy in their 60s or 70s to protect their brain because it might do more harm than good,” he added.

Findings presented at medical meetings should be considered preliminary until published in a peer-reviewed journal.