I think it's harder to get drugs to relieve physical pain - from what I've read it's very painful, headache, stomach ache. You also need to add something to the mental pain: Benzodiazepines etc., so that you don't give up just before the end.
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Antiemetics: Strong enough antiemetics for this method are not OTC, so you'll need to see your way around this. You're looking for Metoclopramide or any of it's commercial names, like Reglan/Primperan. This antiemetic needs to be a Dopamine blocker for it to work.
Antiemetics aren't 100% necessary for this method, however; just like with N, you wont accomplish anything if you end up puking it all out, which is likely. This will also most likely be the bulk of the money you'll use during this method.
Acid Regulators: Another thing recommended in the PPH is raising your stomach's pH to make the SN more effective. This can be accomplished with drugs like Tagamet (The one recommended on the PPH, 800mg, which is to my knowledge OTC) or even bicarbonate, though not as effective.
https://sanctioned-suicide.net/threads/sn-megathread.1156/
I don't know what exactly to take, because I don't plan on taking it, but please ask others because it's very important.