12mg of buprenorphine is more than enough to bring you out of withdrawal. I had a huge heroin habit and I was more than fine inducing to bupe with 8mg.
If you feel like you are still ill after taking that, then you either are using a poor sublingual technique, or you are having anxiety which is increasing the perception of feeling unwell.
Make sure that when you take the subutex, that your mouth is as dry as possible. Using mouthwash and brushing your teeth a few minutes prior to taking it, will increase bioavailability and absorption. Let it dissolve for at least 10-15 minutes, followed by another good 10-15 minutes of swishing the bupe-saliva solution in your mouth like mouthwash, before swallowing.
When I started doing this, I was able to not only feel a very huge difference, but I was able to cut my dose from 8mg to 4mg, while feeling more on 4mg while using this technique, then I felt with 8mg while not using that technique.
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And in reference to someone here trying to advise you to go cold turkey and just stop, it isn't very practical. Of course if we could have, we would have. Saying it is easier than done...and those who have dealt with a heavy opiate habit, has a better chance of making something positive out of their life while being on a maintenance med like buprenorphine, then the alternative which means dealing with PAWS, cravings, depression, anxiety...and everything else which happens physiologically to a person going through PAWS/Post-Acute Withdrawal Syndrome. With opiates...it's not just stopping the substance and resuming normal life. Neurological changes have occurred in the brain which will cause an addict to suffer malaise and other miserable symptoms.
I hope it ends up working out well for you. I've done well with suboxone personally. I had a huge IV heroin habit, between half a gram to a gram daily of eastcoast (Baltimore) powder. And I've been taking suboxone once daily for quite some time and have been able to put aside cravings and build my life back up. There is nothing wrong with doing so, despite others who say its trading one addiction for another, that is completely not the case. My heroin addiction looks absolutely nothing like simply taking a suboxone when I wake up, and going about my day.