While study i am unable to give attention what should i do?

I am studying in my bachelors and i am not able to give attention in my study due to pressure of my coming examination. As i have completed my physiology all the chapter but still i have confusion in that .

As i am living in hostel life and there are four student also living with me and i saw one day one student was using waklerts to give attention in the study and i asked him is that work in attention and he told me the entire story about this and he advide in to buy waklert online Australia but i am not able to afford that cost because i am survive here because i am working as part time some where