In most cases, medicine heals people, or they die despite the medicine. There are rare cases of fatal side effects or drug interactions. Maybe the pharmacist can prevent/minimize these.
If you are referring to intentional overdose, you can’t blame the medicine or the pharmacist or the doctor. They are still there to help people. It is sad that the person who ODed could not be helped, that his pain was so great that he saw no other options. At this point, Sparkly, you need to try to remember that Hashem is in charge and that His plan for us is good, even if we don’t see it at the moment. Also, remember that you are studying to be a pharmacist to help people, to cure disease and alleviate pain and suffering. You may not have been able to help your friend, but you can help others in other ways. You have a purposeful life ahead of you. Try to keep that in mind, and it may reduce the sadness and loneliness you are feeling.