Welcome to Just answer !
Well, yes you may take valium even if you are diagnosed wit Bipolar disorder, but the problem is that valium being a benzodiazepine drug has a potential for getting abused and lead to addiction, and as it is typical of a bipolar disorder , where in the individual develops pleasure seeking behaviour which may lead to abusing prescription drugs to get high/ sense of well being, so the risk for valium abuse may become manifold , so this is one concern that is there with the use of valium with an underlying diagnosis of Bipolar disorder.
So i reckon you should discuss this with your doctor and if at you continue to use valium , then make sure it is either for short duration , say 3-4 weeks only or used on PRN basis that is use as per the need such as in case of severe gad only..
I hope this helps to allay your doubts and gives you a sense of direction to move in.
Wish you all the best.
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