Which of the following is a disadvantage of topically applied sodium fluoride solution?

Advantages of neutral sodium fluoride solution- 1. It is relatively stable when kept in a plastic container and there is no need to prepare a fresh solution for each patient.  2. The taste is well accepted by patients.  3. The  solution  is non-irritating  to the gingiva.  4. It does not cause discoloration of tooth structure.  5. Once applied to the teeth, the solution is allowed to dry for 3 minutes. Thus the clinician in public health programs can pursue a multiple-chair procedure.  6. The series of treatments must be repeated only four times in the general age range of 3  to  13, rather than  at annual or semiannual intervals, therefore in a public health program, other groups of children can be treated in theJintervening years.  Disadvantage of neutral sodium fluoride solution- 1. The major disadvantage of the use of sodium fluoride is that the patient must make four visits to the dentist within a relatively short period of time.