Is being arrested for driving under influence of a controlled substance going to make a difference for a repeat offender?

If this case is in California and this is his seventh within the past ten years he is looking at a very serious case, a felony. He can be prosecuted for DUI even if he was taking only his prescribed medications and at the dosages prescribed. He will need legal representation and the help of a forensic toxicologist and possibly a pharmacologist to avoid prison.

A prescription is irrelevant to driving under the influence.

Your question requires an attorney consultation. It is not a simple question that can be answered on this type of forum. There are many factors that would need to be considered and evaluated. The short answer is probably not, depending on the prescription, it's side effects and what he knew about the drug. A DUI means driving under the influence of alcohol and/or drugs. It does not matter whether it is a prescription or street drug. I strongly suggest that your boyfriend contact an experienced criminal defense attorney for a face-to-face consultation and give him/her all of the facts surrounding his case. He/she would then be in a better position to analyze your his and advise him of his options.

Yes. Drug DUIs are a lot more difficult to prove, and none of it would matter if your boyfriend doesn't have a good DUI specialist representing him.