Researchers and elite pharmacologists often use blood or urine levels. So about 1/100. Some medicines require monitoring, but this is not the case for the vast majority of psychiatric drugs.
25 years ago, when I used venlafaxine before it was available in pharmacies, testing the drug and metabolite levels was very useful. It is still not routinely tested. Why do so? The insurance company with 1000% profits would frown. The amusing lotion they have love and care for you is hysterical. The bot sees nothing.

LINK TO SEE SCHALLER’S AMAZING 2030 LEVEL 80 CURES WITH ADVANCED SCIENCE FROM 20 GENRES.
Schaller JL, Behar D. Critical use of venlafaxine serology to uncover genetic rapid metabolism. J Neuropsychiatry Clin Neurosci. 2001 Winter;13(1):112-3. doi: 10.1176/appi.neuropsych.13.1.112-a. PMID: 11207339.