ITYM Clonidine[1]. There's also Lofexidine[2] which is available on prescription in the UK, but seems largely unknown elsewhere. From personal experience (quitting long-term morphine treatment due to intolerable side effects), it was extremely helpful in moderating some of the physical withdrawal symptoms.
[1] https://en.wikipedia.org/wiki/Clonidine
[2] https://en.wikipedia.org/wiki/Lofexidine