The goal of this trial is to investigate whether low daily doses of fluoridated milk, in addition to oral hygiene routines with fluoridated toothpaste, can prevent caries development in adolescents' teeth. The main questions it aims to answer are:

• Does fluoridated milk lower the number of caries lesions in teeth over time?
• Does fluoridated milk lower the number of enamel caries lesions over time? Researchers will compare an intervention group receiving doses of fluoride in water to add to milk and a control group receiving water to add to milk.

The 12-13 year participants will:

• be examined at a baseline and once yearly up to 2 years
• be provided directly with the doses every second month at their dental clinic
• take the fluid daily

During the study, the participants were asked to continue with their usual oral hygiene regimes, including tooth-brushing twice a day with toothpaste containing fluorides as recommended by the Swedish National Board of Health and Welfare.

At their yearly visit to their dental clinic the participants were asked about their health status, use of antibiotics, lactose intolerance, daily consumption of milk, tooth brushing frequency, type of toothpaste and use of other fluoride additives. Information on adverse effects (dental pain, visit to other dental clinic, and antibiotics due to dental infection) was collected.

Caries registration was performed by visual-tactile and radiographic examinations at baseline and 1 and 2 years. Visual-tactile examination was performed by a dental practitioner under lighting with a mirror and occasionally with a probe. Caries status was classified for the occlusal/incisal, buccal and lingual surfaces and the proximal surfaces of incisors and cuspids according to the ICDAS criteria with code 0 (for sound), 1-3 (for enamel caries) and 4-6 (for dentine caries). ICDAS is an internationally applied system for caries detection used clinically (The International Caries Detection and Assessment System (ICDAS): an integrated system for measuring dental caries (reference). Radiographic examination of the proximal surfaces of premolars and molars with one bitewing radiograph on each side of the mouth took place in the same session.

Inclusion Criteria:

• adolescents of both sexes, aged 12-¬13
• healthy (no oral disease or systemic disease possibly affecting the oral cavity). Health declaration used in the dental clinic was utilized to confirm health status
• adolescents with permanent posterior teeth/no primary molar teeth present and second premolars in contact with the mesial surface of the first molars
• adolescents who, together with their parents or guardians, gave informed written consent to participate. Data confidentiality and the right to withdraw from the study at any time were warranted to the adolescents in writing.

Exclusion Criteria:

None

The subjects and their parents/guardians, the clinicians performing the examinations, the staff-members delivering the packages with solutions to the subjects, the trial investigators and the outcome assessors were blind to the allocation and group assignment. The solutions were distributed to the subjects in tubes pre-labelled with code numbers. The tubes had the same design independent of the solution with fluoride or placebo.

After the randomization, lists with code numbers were saved as sealed files in a safety-deposit box. The code was broken when the study was completed, and the assessment of the outcome measures were processed.