I've heard the first trimester is when the fetus is most susceptible to the effects of medicine. What kind of effects are there?

You need to be particularly careful from the 4th week of pregnancy to the end ofthe 7th week, as your baby's vital organs are forming at this time.
When you get a prescription at a hospital, be absolutely sure to let them know you are pregnant. Be careful to take the medicine in the dosage in which it is prescribed, and by the method ordered. Also, avoid using in great quantities sedatives containing aspirin or any over-the-counter drugs you have purchased at a pharmacy.
Before taking any medicine during this part of your pregnancy, please consult with your obstetrician.