Here are the few tips I can give for buying medicines
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1: always rely on a registered chemist
2: do not be your own doctor and go by what the doctor prescribes
3: do not compensate, or ask for a cheaper substitute of the respective prescription( doctor knows best)
4: check the production and expiry date
5: check the quantity ie donot go for 500 potency if 250 is recommended or vice-versa
6: check with your doctor and the chemist the possible side-effects of the drug or medicine and whether u need a vitamin to combat with it.
7: one can rely on home remedies and the mostly have no side effects for eg: garlic ginger honey brandy curry leaves etc for common colds, fever etc
8:check for any signs of tampering i.e. torn packet or broken seal etc