hello there.i read your query and understand your agony it would have been really helped if you had added a bit more details of yourselves-age,gender,weight,height ,job and its nature etc you also have given no history of any trauma or accidents. coming to your query,i would come to a diagnosis of osteoarthritis of the left knee.zinc tablets alone will be of no use it is basically degeneration of knee joint involving the bone,cartilage and articulating surfaces ,mostly due to ageing process. What i would recommend is take an xray of both knee to assess the condition and extent of osteoarthritis and the following measures: 1)wear a supporting brace for the left knee that would help in reducing movement and thereby pain. 2)try to reduce your own body weight,so that load on knee decreases 3)maintain a healthy diet,with egg whites,milk,a handful of nuts,fruits and vegetables 4)apply analgesic gel like diclofenac or aceclofenac and apply heat over the area 5)take aceclofenac tablet orally after food when pain is severe.avoid unnecessary usage and try to use the gel mostly 5)supplement tablet like glucosamine and chondroitin sulphate with or without diacerin will help in repairing the damaged cartilage 6)you can do exercises which strengthen the thigh muscles all of this will help you for sure.hope this clarifies your doubts.good day