If you take the prescription drug ambien, and you drive a car, you are at risk of driving under the influence of drugs in New Hampshire. It does not matter if you have a valid prescription. The issue is did the ambien impair your ability to drive to any degree. One of the problems with ambien is it can cause people to basically sleep walk. People have been known to take the drug before going to sleep, not remember getting up or driving, and later become coherent after they are already arrested. Combining alcohol with ambien can affect how much it affects you, as well as stress and other factors. If you take ambien you really need to be careful, especially when you first start off taking it. Also,do not exceed what a doctor recommends.
If you are charged with DWI ambien, to present the best defense you are going to need an expert witness to speak of the effects of ambien (as well as a really good DUI Attorney to question everything else). the state crime lab analyst will testify that the ambien in your blood was enough to impair you if you gave a blood test.
Additionally, some cops are drug recognition experts and do a 12 step DRE exam on you. It is necessary to know how the evaluation is supposed to be done, and have an expert testify as to the limitations of the exam.
Finally, it might be a defense that you did not know what you were doing. While voluntary intoxication is never a defense to DWI, it does require a voluntary act. Sleepwalking could be a defense While there is not presently any law on this defenss in NH, recently the Oregon Supreme Court held that sleep driving is a defense in State v. Newman, No. SC S060182. "The trial judge must permit him to put on expert testimony that his driving was not a volitional act because it occurred during an episode of somnambulistic automatism -- sleepwalking (which led to "sleep driving")" "regardless of whether a person has a culpable mental state, that at minimum the person must engage in a volitional act before that act can be considered criminal. Thus, if a person commits the act in his sleep, the act is not volitional and does not subject the actor to criminal liability." - http://www.linkedin.com/groupItem?view=&srchtype=discussedNews&gid=2365795&item=245874999&type=member&trk=eml-anet_dig-b_pd-ttl-cn&ut=0RBh9IbSz3YBM1