Efficacy and Safety of MK-1942 as an Adjunct Therapy in Participants With Mild to Moderate Alzheimer's Disease Dementia (MK-1942-008)

Inclusion Criteria: * Has mild to moderate AD dementia based on the national institute of neurological and communicative diseases and stroke/Alzheimer's Disease and related disorders association (NINCDS-ADRDA) criteria. * Has mini-mental state examination (MMSE) score between 12-22 (inclusive) at screening. * Is using acetylcholinesterase inhibitors (AChEI) therapy for management of AD dementia at Screening and during the study. These medications must be at stable approved dose levels ≥3 months before the first dose of study intervention and the regimens must remain constant throughout the study to the extent that is clinically appropriate. * Has a designated study partner who can fulfill the requirements of this study. The study partner will need to spend sufficient time with the participant to be familiar with their overall function and behavior and be able to provide adequate information about the participant needed for the study including, knowledge of functional and basic activities of daily life, work/educational history, cognitive performance, emotional/psychological state, and general health status. Exclusion Criteria: * Has a known history of stroke or cerebrovascular disease that is clinically important in the investigator's opinion. * Has diagnosis of a clinically relevant central nervous system (CNS) disease other than AD dementia (with protocol-specified exceptions). * Has a history of seizures or epilepsy within the 10 years preceding Screening. * Has any other major CNS trauma, or infections that affect brain function. * Has evidence of a clinically relevant or unstable psychiatric disorder, based on criteria from the diagnostic and statistical manual of mental disorders (fifth edition), including schizophrenia or other psychotic disorder, bipolar disorder, major depression, or delirium. Major depression in remission is not exclusionary. * Has a severe, acute, or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or administration intervention. * Has a history of malignancy occurring within the 5 years immediately before Screening, except for a participant who has been adequately treated for 1 or more of the following: basal cell or squamous cell skin cancer; in situ cervical cancer; localized prostate carcinoma; who has undergone potentially curative therapy with no evidence of recurrence for ≥3 years post-therapy, and who is deemed to be at low risk for recurrence. * Has a risk factor for QTc prolongation. * Has a history of alcoholism or drug dependency/abuse within the 5 years preceding screening. * Has a known allergy or intolerance to the active or inert ingredients in MK-1942. * Has received any anti-amyloid agents or antibodies, or any of the following medications: CNS-penetrant anticholinergics, neuroleptics, anticonvulsants, narcotics, glutamatergic agents, agents with possible psychotropic effects, and experimental acute respiratory syndrome coronavirus 2 (COVID-19) therapies. * Has liver disease, including but not limited to chronic viral hepatitis, non viral hepatitis, cirrhosis, malignancies, autoimmune liver diseases. * Has an abnormal thyroid-stimulating hormone (TSH) value if confirmed by abnormal T4 value. * Resides in a nursing home or assisted care facility with need for direct continuous medical care and nursing supervision. Participant may reside in such facilities provided continuous direct medical care is not required and a qualified study partner is available for coparticipation and the participant is physically able to attend all required study visits. * Had major surgical procedure or donated or lost \>1 unit of blood (approximately 500 mL) within the 4 weeks before screening.