Most of the time, I feel sick (due to the side effects) and I cannot study much after the class.
I have not been keeping well (due to the side effects of medication I am on), and therefore I cannot study much after the class.
薬の副作用ーSide effects of the medicines.
薬の副作用で吐き気するーI feel like vomiting due to the side effects of medication I am having right now.
癌の治療には薬の副作用が絶えられないぐらいしんどいよーThe medicines used for Cancer treatment have many side effects and sometimes it becomes really difficult to make it through the entire process.
以上は副作用に関する英語の表現になります。
どうぞご参考までに。