Lithium and Opiates?

I just want an answer from an EDUCATED person who can site sources to back what they say, please. If you have first hand experience, please respond as well.
Ok, 2 weeks ago I was started on Lithium Extended Release tablets. 450mg, 2x a day. So 900mg a day. I've been doing this for 2 weeks now and recently whenever I try to use an opiate, it seems as if I need much more than normal to get high. And I don't get that high unless the dose is VERY high. Ex: I snorted 20mg OxyContin which usually gets me high enough to nod off. However, I barely felt it. Is this due to the Lithium or something unreleated? And if so, what can be done about this so I can get high off opiates with normal dosing? Any info or advice whatsoever is appreciated. Thank you.

Lithium should not make you less sensitive to opiates. I strongly suspect that the two issues are unrelated.

i know this thread is old as hell, but fairnymph is completely wrong.

lithium binds, and ultimately blocks, the same receptors in your brain that wanna bind with opiates! so you WILL have to do more of any kind of opiate if you wanna get that same feelin you had before you started taking lithium on a regular basis.

IME, lithium screws with a lot of other shit too.