I took concerta xr 18mg for adhd at 8 a.m. is it ok to have one drink at night since theoretically the concerta wears off in less than 12 hours?

Unclear: This is a highly individual matter that involves multiple factors, not just the dosage or the half-life or the duration of action of the medicine. You need to talk to the prescribing doctor.

Alcohol ?: My rule of thumb for all medications, especially psychoactive ones, is simply - no alcohol if you are taking a medication, the side effects could be devastating !!