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?