At your age the most common cause for knee pain is a tear in the meniscus or softening of the cartilage behind the knee cap (patella). In both the conditions meds will not help. You have to reduce certain activities in order to change the loading pattern on the knee. First of all, try to avoid squatting or sitting cross legged on the floor as far as possible. Next avoid climbing steps. Third, if you are into exercises, avoid those involving running and jumping. Do non-impact exercises like walking, cycling or swimming. Wearing a knee cap can help as well. Visit a good physiotherapist who can teach you exercises to unload the knee. Meds are required to reduce inflammation in the knee for about 10 to 14 days, not more than that. If possible get an xray of the knee joints done and visit an ortho surgeon if your pain persists beyond 3 weeks. For a second opinion mail the xrays to bdbaruah[at]orthodoc[dot]in