Phenolic diterpenes—particularly carnosic acid and carnosol—are compounds found in herbs such as rosemary (Rosmarinus officinalis) and sage (Salvia officinalis). Their potential role in supporting or treating Alzheimer's Disease (AD) has been explored in preclinical studies due to their antioxidant, anti-inflammatory, and neuroprotective properties. Laboratory and animal studies have demonstrated that phenolic diterpenes can reduce oxidative stress, inhibit amyloid-beta aggregation, and protect neuronal cells from damage—all mechanisms implicated in the progression of Alzheimer's Disease. For example, carnosic acid has been shown in animal models to activate the Nrf2 pathway, leading to enhanced cellular defense against oxidative injury, which is considered relevant to neurodegenerative disorders like AD.

However, the bulk of evidence is currently preclinical. There is a lack of robust, high-quality human clinical trials specifically evaluating phenolic diterpenes for the treatment or prevention of Alzheimer's Disease. While some small clinical studies with rosemary and sage extracts (which contain these compounds) have shown modest cognitive benefits in older adults or those with mild cognitive impairment, these studies seldom isolate phenolic diterpenes as the active constituents. As such, the scientific evidence supporting their use in Alzheimer's Disease is limited (rated 2/5), and more rigorous human trials are needed before definitive conclusions can be drawn.