You are legally on a prescription or even an over-the-counter medication. You are driving your vehicle. Can you be pulled over, cited, and convicted of a driving under the influence (DUI) charge?
The answer is yes, if you are in fact under the influence and driving!
Driving under the influence, Vehicle Code Section 23152, provides: "It is unlawful for any person who is under the influence of any alcoholic beverage or drug, or under the combined influence of any alcoholic beverage and drug, to drive a vehicle."
So what constitutes a drug? According to California Vehicle Code Section 312: "The term "drug" means any substance or combination of substances, other than alcohol, which could so affect the nervous system, brain, or muscles of a person as to impair, to an appreciable degree, his ability to drive a vehicle in the manner that an ordinarily prudent and cautious man, in full possession of his faculties, using reasonable care, would drive a similar vehicle under like conditions."
The fact that you are legally entitled to use the drug is not a defense to a DUI case, per California Vehicle Code Section 23630.
According to the DUI jury instructions (2110), you are under the influence "if, as a result of...taking a drug...[your] mental or physical abilities are so impaired that [you are]...no longer able to drive a vehicle with the caution of a sober person, using ordinary care, under similar circumstances. The manner in which a person drives is not enough by itself to establish whether the person is or is not under the influence of...a drug...However, it is a factor to be considered, in light of all the surrounding circumstances, in deciding whether the person was under the influence."