I don't know where you are, but I don't think you will be able to just go to the doctor and request a specific medication. You tell the doctor your symptoms and they choose your medication accordingly. The only possible exception to that is if you have previously had a particular medication and had a bad reaction to it, it didn't work, it made you worse, you have medical reasons why specific ones are bad for you etc. and the doctor has previously told you a particular one is the one you should take, and you therefore request that. Having said that, doctors are people too and maybe some are more open-minded to discussing with the patient particular medication. It's their decision though as the buck stops with them, and many doctors don't like well informed patients that turn up wanting to ask for something specific. It messes with their God complex.
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*Truth fears no trial*
DX AS & both daughters on the autistic spectrum