Hi,
Hope you are doing well.
The post of drug inspector is very common in health ministry whether it is central or state, various awareness programmes and even inspection of business units in cosmetics, medicines, storage of food items etc.
After pursuing your graduation in B.pharmacy ,you can have working experience in any pharmacy medication company. Many states ask for a 18 months work experience to get eligible for the post of drug inspector.

UPSC( Union Public Service Commission ) and various SPSCs ( State Public Service Commission) releases notification and vacancies from time to time.
You have to pass in the written exam held by UPSC or SPSCs followed by the interview process to become a drug inspector.

Hope it helps.

Best Wishes