My Age Is 17 Yrs. My First Period Was At

My age is 17 yrs. My first period was at the age of 10 yrs. every month during periods I get severe cramps. I take 2-3 meftal spas tablets per day for 1st 2 days. but onset of action is almost after 2.5 hrs. I normally miss my classes because of this and I feel sleepy after taking tab... . Now I have 12th exam in feb 16. How should I deal with it. Can I take tablet before pain starts. Is there any other alternative for fast action? Pls. advise.