Medication can only be prescribed by health professional registered to do so; typically a Psychiatrist or Paediatrician.
I have heard mixed reviews on the use of medication in the treatment of SM. All agree that medication should ONLY be used as part of a comprehensive treatment programme and not in isolation and ONLY when less invasive interventions have been trialled first.
In some cases I have seen medication used successfully to enable a child to overcome their anxiety enough to engage in a more behavioural intervention programme. In other cases the use of medication has been advised against due to the possibility of reliance on these external supports.
The long term side effects of medication are still yet unknown and as such I would advocate that any decisions in this area are considered with a medical team who have experience with using the medications prescribed and only after other interventions have been trialled.
The following links provide some information and research in this area:
http://www.selectivemutism.org/resources/library/index_html/medication%20issues/ - research by Dr Kurtz on medication and SM