new patients only use 18% approximately of SSRI's prescribed -

Figure 4 (below) translates medication doses of paroxetine,
fluoxetine and sertraline into patients and demonstrates
that the number of new patients is not a
simple proportional function of the scripts and pills
issued. In fact the number of new patients tends to
stabilize at around 300,000 per year for all three pills
combined. The distribution of medication between
new and long-term patients is shown in Figure 5.
By 2003, new patients only use 18% approximately
of the medication issued for these three drugs.

http://psychrights.org/research/Digest/AntiDepressants/HealyAldred2111.pdf