What is the legal effect of driving while impaired by over-the-counter or prescription cold/allergy medications that cause drowsiness?

1. It is fully legal as long as you have a doctor's receipt in the car
2. It only counts as an offense if mixed with coffee
3. It is permitted if you drive below 15 mph
4. It is illegal; driving while impaired by any drug or medication that impairs driving ability is an offense under the law