Naltrexone is an opiate antagonist. It was developed to combat narcotic
addiction and is now widely touted as helping to reduce the craving for
alcohol in those addicted to that substance.
Some people believe that people with autism have an excess of opioids in
the body and that they indulge in self-hurting behaviour because the
physical pain involved increases the secretion of endorphins, opioid
substances manufactured in the body. The reasoning goes that if the
opioid receptors are blocked by naltrexone, self-hurt will result in no
satisfaction and will therefore stop.
In practice, naltrexone treatment for autistic people who hurt
themselves has been something of a disappointment. It works for some,
but then it stops working after a while. Others have found that its
undesirable effects, like headache, nausea, dizziness, tiredness,
insomnia, anxiety or sleepiness, are impossible to live with.
I honestly think it's worth trying, though, for somebody who seems to be
at continual risk from self-harm.