L-methylfolate: If patient has depression resistant to standard treatments *and* either documented MTHFR mutation or elevated homocysteine
Vitamin D: If patient has depressive symptoms *and* serum 25-OH-D below 30
Melatonin: For short-term rebalancing of circadian dysrhythmia
N-acetylcysteine: For trichotillomania or skin picking
I usually put in a prescription unless the patients tells me the medication is easier/cheaper to find OTC. NAC in particular is not carried by many pharmacies but available cheaply on Amazon.
I have had patients come to me who request to take things like St John's Wort, Sam-E, ashwagandha, and various Chinese preparations that I know nothing about.
In general if they are already taking it and finding benefit I don't demand that they stop it, but I do ask them to please choose either an OTC supplement or a prescription, and not to take both together, since many of these may have overlapping mechanisms of action and could lead to serotonin syndrome or co-interference with drug metabolism.
Herbal preparations especially are almost totally unregulated and their content and potency can vary widely. There have been numerous publications documenting enormous discrepancies between the label claims and the actual content; for example this one, in which DNA from the source plant listed on the label was detected in only 4 out of 15 samples.
Authentication of Herbal Supplements Using Next-Generation Sequencing