Got this off the LDN website
Are there any side effects or cautionary warnings?
> Side effects:LDN has virtually no side effects. Occasionally, during the first week's use of LDN, patients may complain of some difficulty sleeping. This rarely persists after the first week. Should it do so, dosage can be reduced from 4.5mg to 3mg nightly.
> Cautionary warnings:
Because LDN blocks opioid receptors throughout the body for three or four hours, people using medicine that is an opioid agonist, i.e. narcotic medication — such as Ultram, morphine, Percocet, Duragesic patch or codeine-containing medication — should not take LDN until such medicine is completely out of one's system. In addition, LDN should probably not be taken during pregnancy.
Full-dose naltrexone (50mg) carries a cautionary warning against its use in those with liver disease. This warning was placed because of adverse liver effects that were found in experiments involving 300mg daily. The 50mg dose does not apparently produce impairment of liver function nor, of course, do the much smaller 3mg and 4.5mg doses.