Oral hygiene is difficult to achieve for frail older adults. Aging, chronic diseases, polypharmacy, mouth-washes, and crushed drugs can contribute to uncontrolled proliferation and microbial deposits in the mouth. Looking for avoidable risk factors, in vitro microbial survival or proliferation in thickened drinks and oral nutritional supplements (ONS) was investigated. The safest thickened drinks were ready-to-use products containing preservatives. Escherichia coli , Staphylococcus aureus, and Candida albicans proliferated in dairy ONS at room temperature. C. albicans also proliferated in juices. Oral anerobic bacteria were recovered from part eaten ONS. Thickened drinks and ONS could contribute to microbial proliferation, especially with patients who have swallowing alterations or cognitive troubles, who may keep these solutions longer than necessary in their mouth. These products can also constitute microbial reservoirs in the environment of frail older adults. It is important for health care workers and family members to respect hand hygiene and refrigeration procedures. [ Research in Gerontological Nursing, 13 (4), 203–209.]