In summary, there is no convincing evidence to say that alcohol intoxication helps to treat or prevent parasites in humans.
1) The evidence from in vivo human studies does not support the idea that alcohol consumption helps in treating parasites.
Alcoholism and Strongyloides stercoralis: Daily Ethanol Ingestion Has a Positive Correlation with the Frequency of Strongyloides Larvae in the Stools (PLoS, 2010):
The frequency of Strongyloides was significantly higher in alcoholic
patients than in control group (overall prevalence in alcoholic 20.5%
versus 4.4% in control group; p = 0.001).
2) Even a strong alcohol beverage gets diluted when it reaches the intestine.
Ethanol concentrations in the human gastrointestinal tract after intake of alcoholic beverages (European Journal of Pharmaceutical Sciences, 2016):
In a cross-over study, five fasting volunteers were asked to drink two
standard consumptions of commercially available alcoholic beverages,
including beer (Stella Artois®, 500 mL, 5.2% ethanol), wine (Blanc du
Blanc®, 200 mL, 11% ethanol) and whisky (Gallantry Whisky®, 80 mL, 40%
The median gastric ethanol Cmax (min–max) for the beer, wine and
whisky conditions amounts to 4.1% (3.1–4.1), 4.1% (2.6–7.3) and 11.4%
(6.3–21.1), respectively...Median duodenal ethanol Cmax (min–max) for
beer, wine and whisky are 1.97% (0.89–4.3), 2.39% (2.02–5.63) and
5.94% (3.55–17.71), respectively.
So, the maximal ethanol concentration in the duodenum after drinking 80 mL of whisky was 17.7%. Most of ethanol is absorbed in the first part of the small intestine (Scandinavian Journal of Gastroenterology), so it does not reach the more distant parts and does not likely kill the parasites there.
3) Even strong intoxication is associated with low blood alcohol concentration.
In strong intoxication, your blood alcohol concentration would be only 0.2-0.3%. In one study (Table 1), the 50% lethal concentration (LC50) of ethanol, which killed 50% of the bloodstream forms of the parasites Trypanosoma brucei, was 10.6%.
4) Alcohol intoxication and brain dehydration
Alcohol intoxication or hangover are not automatically associated with dehydration. Anyway, even in severe dehydration, you still have a lot of water in your body, including the brain, so the parasites living there do not necessarily get dehydrated as a result of your dehydration.
In general, dehydration increases the risk of infections, because it dries mucous membranes, for example in the urinary tract (BMJ Open Quality, 2019).