In regards to acetaminophen overdose, consider body weight, metabolism, and the health of your heart and liver; however, considering a lethal or toxic dose is typically when you meet or exceed 12g (12000 mg) or more in one setting (usually within an 8 hour span, but elongated to 18000mg in a 24 hour). That would be 24 tablets of acetaminophen (500 mg each) or more, which is classified as the maximum dosage you should consume (if at all) before damage to the liver or kidneys commences. I have had many people ask me this when I worked in the pharmacy, but as a CPhT, I can honestly say that due to not knowing medical conditions, it would be rather difficult to say what would be YOUR exact dosage lethality, but i would hold it to an approximate of 22 tablets for someone your size. NOW concerning the extensiveness of Dextromethorphan (DXM), you need to understand that it has multiple properties and purposes other than a cough supressant, and that when used in conjunction with other drugs, (primarily promethazine, amphetamines, and although controversial, NSAID affiliates such as bayer aspirin and the like) it can reduce the immunity to certain disorders and viruses. Just keep in mind that if you are going to Robotrip, flip Triple C's or whatever use you have to procure your use of Dextromethorphan, keep in mind also that the extensive research of DXM is not entirely thorough in it's medical evaluation, so there is still alot of speculation regarding the health risks when procuring its repeated use.
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FINAL NOTE:
If you're going to use Dextromethorphan, do not use it repeatedly for the use of getting high, if that is what you are going for, as there are much more "safe" (and I say that with caution), ways to expand your universe and your mind than destroying your organs with DXM and Acetaminophen. Happy hunting, and stay safe whan you experiment with anything! No reason to die for a high. I have had one of everything out there so far, and there is NOTHING good enough to want to die for lol. Much respect for you asking.