There is no open source but there are few good available at very reasonable rates.
Do you own a drug store (Medical store) or you want to redevelop it and sell it forward?
My bro is a chemist and he is using one.
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This will not work for pharmacy.
Well you can make it work but I don't think this is what the OP has in mind when saying Pharmacy software. Pharmacy s/w generally emphasis more on inventory/ drug databases / predefined reports to help with pharmacy management
and integration with wholesalers/distributors for automated inventory management.
Retail invoicing and inventory can be managed by any accounting or pharmacy software.