Buspirone, a non opiate drug with no abuse potential, a safe side effect profile and no withdrawal symptoms at doses of 30 and 45 mg, was as effective as a methadone taper in alleviating the withdrawal symptoms of heroin addicts stabilized for 5 days with, and then withdrawn from, methadone.
This study http://www.ncbi.nlm.nih.gov/pubmed/2956114 suggests that 5-HT1a agonists increases secretion of beta-endorphin, and there are others that suggest it's helpful in withdrawal from all sorts of drugs.