More than 5.5 million Americans are currently living with Alzheimer's disease. It's the sixth leading cause of death, and its numbers of victims are growing. The Alzheimer's Association estimates that by 2050, the number of Alzheimer's patients could reach 16 million.

Currently, there is no cure, but several drugs have been approved to improve cognition. But which medications are the most effective? Researchers at Ontario's St. Michael's Hospital compared the safety and effectiveness of the four most widely prescribed drugs to treat moderate to severe Alzheimer's dementia: donepezil, rivastigmine, galantamine or memantine.

"Alzheimer's dementia is the most common form of dementia in North America, and most people who have moderate to severe Alzheimer's will be on these medications," said the study's lead author Dr. Andrea Tricco, a scientist in the Li Ka Shing Knowledge Institute of St. Michael's Hospital.

The study reviewed 142 clinical trials of the four common cognitive enhancers administered alone or in combination published between 1996 and 2015. The number of patients in each study ranged from 13 to 2,045, and the review evaluated a total of 33,889 patients.

The researchers compared the safety and effectiveness of any combination of donepezil, rivastigmine, galantamine or memantine in treating moderate to severe Alzheimer's dementia based on the results of the clinical trials that examined a number of patient outcomes, including cognition, function behavior, global status, mortality, serious adverse events, falls, bradycardia, headache, diarrhea, vomiting and nausea.

Donepezil was likely the most effective medication for Alzheimer's dementia across all effectiveness outcomes, including cognition, behavior and overall health, according to the study.

Donepezil was also the only cognitive enhancer that reached the minimal clinically important threshold — meaning effects on outcomes were observed clinically, as well as statistically — on the Alzheimer's Disease Assessment cognition scale, making it the likely first choice for those patients and clinicians considering these medications, the authors said.

"This analysis will give both patients and clinicians a full picture of how each of these drugs will likely affect their cognition, as well as their overall health," said Tricco.

"The more information we are able to gather about how each of these medications can affect a patient's cognition and health, the more likely we are to be able to improve their health outcomes," she said.

The report was published in the Journal of the American Geriatrics Society.