I have never heard of anyone taking higher than the 4.5mg.....
according to the lowdosenaltrexone.com website, dosages above 4.5mg are likely to block endorphins for too long.....and interfere!!

"The therapeutic dosage range for LDN is from 1.5mg to 4.5mg every night. Dosages below this range are likely to have no effect at all, and dosages above this range are likely to block endorphins for too long a period of time and interfere with its effectiveness"