All states have two types of DUI charges: "per se" charges and charges based on actual impairment. Proving a per se charge involves showing the driver had a blood alcohol concentration (BAC) of .08% or more or a certain amount of drugs in his or her system. (So, defending against a per se charge often involves disputing the accuracy of a blood- or breath-test.) With impairment DUIs, on the other hand, the prosecution focuses on the actual effects that the drugs or alcohol had on the driver.
State laws differ on the level of impairment prosecutors must show to get a DUI conviction. In some states, like in Maine, the required showing is minimal: The prosecution just needs to prove that drugs or alcohol had some effect on the driver. In other states, such as California, the substances ingested must have had a "substantial" effect or rendered the driver "incapable" of driving safely. But regardless of the impairment standard, the prosecution generally must link the driver's condition to drugs or alcohol.
It can be tricky to convince a jury that you weren't impaired enough to meet the legal standard for a DUI—especially in states that just require proof of "some" effect. Evidence of impairment often comes from the arresting officer's observations. For example, the officer might testify that you:
To counter the officer's testimony, the best tact is usually to provide an innocuous explanation for the officer's observations rather than directly dispute what the officer observed. For example, allergies and certain medical conditions can cause bloodshot eyes. And a disability or an injury might explain less-than-perfect FST performance. Of course, for some observations (like the odor of alcohol), there may not be an innocent explanation.
In some cases, an officer might wrongly assume a driver's odd behavior is related to drug or alcohol use. For instance, mental illness can be difficult to distinguish from intoxication. If there are no test results showing an amount of alcohol or drugs in the driver's system sufficient to cause impairment, the driver might be able to persuade the jury that the apparent intoxication was actually related to something other than drugs or alcohol.
Having a prescription for a drug or medication generally won't get you off the hook for a DUI. However, some states, like Minnesota, allow a limited defense for drivers who can show they took a prescription medication as prescribed by their doctor.
DUI laws vary by state, and the facts of each case are different. So, to find out if you have any viable defenses, it's always a good idea to get in contact with a knowledgeable DUI attorney in your area. A qualified lawyer can tell you how the law applies to the facts of your case.