Does there need to be a comma before as in any of these statements?
Return to the clinic as recommended.
She will return to the clinic as needed.
She has a Baker's cysts as described above.
Antipsychotics as per the treating physician.
If there is no comma, could you explain to me in dummy terms, why?
The only one which perhaps could have a comma is the last one. The rest do not need one - the phrases containing "as" are contingent on the preceding word/phrase and are related to a continuing state.