From what I can gather, the reason such high doses are necessary is because of the enyzme Cytochrome P450 3A4, which almost completely metabolizes loperamide during first pass, at normal doses preventing the majority of it from reaching systemic circulation. I'm assuming you drank grapefruit juice or something similar to inhibit that enzyme's ability and thus allowing loperamide to reach systemic circulation. Then, it's next obstacle is it's inability to cross the blood brain barrier well due to the transporter p-glycoprotein. High doses are required to overwhelm it and cross the BBB, but once it reaches the brain, it appears to act no different than any other exogenous opioids in terms of neurotoxicity or brain damage. I can't find much that would suggest otherwise, but I'd be interested to learn further.