Most opiates (including hydrocodone, oxycodone, heroin, morphine and hydromorphone) are non-toxic physically and, as far as I've seen and read, the only possible neurotoxicity would be from oxygen deprivation from nodding off on them, which I find near impossible with hydrocodone anyway.
Tolerance will get to be a serious bitch though, I'm not a daily user, I generally use opiates once or twice per week although it depends on the time, sometimes I go for a month without any opiates and sometimes I use 4 or 5 days in a single week. Anyway, with that pattern of use I have to take around 100-150 mg of hydro to get a nice high going. But my health is fine.