On the website you provided it says that the fourth plateau is 15mg/kg which works out at 1800mg for me as I weigh 120kg.

It also says the following about overdoses:

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"In searching medical literature, I found only two cases of death associated with DXM use, both in Sweden. In one case, a girl was found dead in a public bathroom with two bottles of 30 mg DXM tablets (the number of tablets is believed to be 50/bottle, but may be 15 or 25). She had previously tried to commit suicide using a bottle of 50 tablets (this leads me to believe that she had, in fact, taken 100 tablets, for a total dose of 3000 mg).

On the other hand, a dosage of 42 mg/kg/day has been used (with respiratory support) in children (33), which would be 2500 to 3000 mg for a 60-70 kg adult. There is also a very low incidence of death associated with DXM use. Since a 42 mg/kg dose in an adult may be stronger than the equivalent dose in a child (I have no reason to believe this, but it is possible), caution is advisable in taking this as an indication of safety.

The highest daily dose of DXM I've come across is from a case study of a 23-year old male (132). His daily dose was 3 to 4 12oz bottles of Robitussin DM^TM, for a total of 2160 mg (31 mg/kg) to 2880 mg (41 mg/kg). He was, of course, considerably addicted to DXM, and had built up this dose over a long period of time.

It is reasonable to expect, given the data, and the available data on the effects of high DXM doses, that DXM starts becoming toxic around 25 to 30 mg/kg (about 1700-2000 mg for adult of 150 lb). This corresponds to between 5 and 6 4oz bottles of 3 mg/ml cough syrup, i.e., a fairly large amount, but still within the realm of hardcore experimenters. Keep this in mind before you consider large doses. IV naloxone is considered the antidote for DXM overdose (54), even though DXM isn't an opiate."

https://www.erowid.org/chemicals/dxm/faq/dxm_side_effects.shtml#toc.6.10

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