LDN is still considered as "alternative" medicine- so some doctors dont like to prescribe it, but because is used for number of different issues any doctor can prescribe it. Its patent expired so its available as generic tough its hard to find- but you have te be aware that naltrexone would not work- it needs to be low-dose-naltrexone; other thing is it seems that only slow-release LDN is giving good results.
Website dedicated to LDN: The Low Dose Naltrexone Homepage
Some extracts: The usual adult dosage is 4.5mg taken once daily at night. Because of the rhythms of the body's production of master hormones, LDN is best taken between 9pm and 3am. Most patients take it at bedtime.
Notable exceptions:
People who have multiple sclerosis that has led to muscle spasms are advised to use only 3mg daily and to maintain that dosage.
Another site from UK which also contains some studies and research, I found those from Stanford LDN research very interesting: http://www.ldnnow.co.uk/47401.html