OP, I'm going out on a limb here, as I'd never even heard of this drug before you mentioned it in this thread. I see a lot written regarding it's similarity to Clonidine, which is prescribed commonly for withdrawal of all kinds here in the US. This is a good thing. Now, assuming it is similar to Clonidine in effect, here is what you can expect. A lot of the more pesky symptoms of withdrawal can be somewhat mitigated by the use of these drugs. Sweats, chills and high blood pressure are all symptoms of opioid withdrawal and they can be quite annoying, but my experience with Clonidine is that is helps relieve this overall "tense" feeling that I experience in withdrawal.
You've stated that you're not interested in Methadone or Buprenorphine, so I suppose this drug is a logical choice. Do not expect it to take away your withdrawal. Opioid withdrawal is a harrowing experience, but it will help a little bit and make you more comfortable. I think you should definitely give it a try. Can I ask, are you or your prescriber opposed to benzodiazepines? They can help quite a bit and might even enable you to sleep. They are obviously not related to opioids, so they can be used without affecting or prolonging your withdrawal.
If you can use benzodiazepines responsibly, then I would recommend requesting them as part of your withdrawal regimen. Diazepam (Valium) and Chlordiazepoxide (Librium) are both sensible choices as they are long lasting and can give you some long lasting and stable relaxation, or, at least as close to relaxation as you can get in withdrawal. Sorry to go off topic, I just thought this suggestion might be pertinent to your issue. Good luck and I hope the experience isn't too bad for you. 900mg of Codeine is not a severe habit, but it's all relative and every person experiences withdrawal in their own way.
