Medical opinions gain sway through quantities of drugs (9)

'medical opinions' is the definition.
'diagnoses' can be an answer for 'opinions' (a diagnosis is a medical opinion). I am not sure about the 'medical' bit.

'gain sway through quantities of drugs' is the wordplay.
'sway' indicates an anagram (I've seen 'swaying' mean this (letters swaying or moving around)).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'quantities of drugs' becomes 'doses' (I can't justify this - if you can you should give a lot more credence to this answer).
'gain' anagrammed gives 'iagn'.
'iagn' put within 'doses' is 'DIAGNOSES'.

Can you help me to learn more?