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Background Bacopa monnieri, a herb that has been used for many centuries in India, has shown neuroprotective effects in animal and in vitro studies; human studies on patients with Alzheimer disease have been inconclusive. Objective The primary objective of this review was to determine the clinical efficacy and safety of B. monnieri in persons with mild, moderate, or severe dementia, or mild cognitive impairment, due to Alzheimer disease. Methods We searched PubMed, Embase, Cochrane Library, clinical trial registries (World Health Organization, Australia-New Zealand, United States, and South Africa), the metaRegister of Controlled Trials, and CINAHL. We intended to include all randomized and quasi-randomized controlled trials that compared B. monnieri, its extract or active ingredients (at any dosage), with a placebo or a cholinesterase inhibitor among adults with dementia due to Alzheimer disease and in those with mild cognitive impairment due to Alzheimer disease. Results Our comprehensive search yielded 5 eligible studies. A total of 3 studies used B. monnieri in combination with herbal extracts while the remaining 2 used B. monnieri extracts only. Two studies compared B. monnieri with donepezil while the others used a placebo as the control. There was considerable variation in the B. monnieri dose used (ranging between 125 mg to 500 mg twice daily) and heterogeneity in treatment duration, follow-up, and outcomes. The major outcomes were Mini-Mental State Examination scores reported in 3 trials, Cognitive subscale scores of the Alzheimer’s Disease Assessment Scale in 1 study, and a battery of cognitive tests in 2 studies. Using the Cochrane risk-of-bias tool, overall, we judged all 5 studies to be at high risk of bias. While all studies reported a statistically significant difference between B. monnieri and the comparator in at least one outcome, we rated the overall quality of evidence for the Alzheimer’s Disease Assessment Scale-Cognitive Subscale, Postgraduate Institute Memory Scale, Mini-Mental State Examination, and Wechsler Memory Scale to be very low due to downgrading by 2 levels for high risk of bias and 1 more level for impreciseness due to small sample sizes and wide CIs. Conclusions There was no difference between B. monnieri and the placebo or donepezil in the treatment of Alzheimer disease based on very low certainty evidence. No major safety issues were reported in the included trials. Future randomized controlled trials should aim to recruit more participants and report clinically meaningful outcomes. Trial Registration PROSPERO CRD42020169421; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=169421