How bad can overdosing be if I take a medicine after 12 hours instead of 24 hours?

[Rafi1]

I have pulmonary hypertension due to a heart defect called aortopulmonary window. I take letairis (ambrisentan) once a day. Sometimes I miss a dose and I feel horrible. What is worse is that sometimes I can't remember whether I took the last dose. Even though I feel horrible inside I still wait until at least 16-18 hours for taking the tablet again because of fear of overdose. Today I can't remember whether I took letairis last night. 8 hours have passed since last night. I don't want to wait 16-18 hours to take the dose. If I took it last night how bad can the overdosing side effects be if I take it again after 12 hours? I think some medicines last only 12 hours in blood stream but doctors tell you to take it again after 24 hours to let the organs which are affected by its side effects heal to prevent long term organ damage. But it might be okay to take it  after 12 hours if you are not sure whether you took your last dose. Should I take it after 12 hours from now on if I am not sure whether I took the last dose?

[erijon]

letairis has a half life of 15 hours meaning every 15 hour one half of the amount of the drug in your system goes away. By the time you took it again at 12 hours you still had over half the concentration in your blood. You need to stick to your doctor's orders. The manufacturer recommends you skip the missed dose if it is near the next dose. In this case I would call the doctor's on call representative and get dosing instructions.

[GardeniaDreams]

Maybe a pillbox fill once a week and as backup post-it notes write date/time for med, after you take pill you discard/shred post-it note.