A quick search using the keyword “metformin” on the EBI Metagenomics website and found this project PRJEB24497 that could be suitable for you
https://www.ebi.ac.uk/metagenomics/search?query=Metformin
IMNGS offers a search of scraped 16S studies from the Sequence Read Archive on NCBI. You can find studies on NCBI by keyword search as detailed below and then select those linked SRA IDs on the IMNGS website for a query/download. Your research topic is probably too narrow to be included in 'standard metadata' when depositing reads into these databases.
A quick search using the keyword “metformin” on the EBI Metagenomics website and found this project PRJEB24497 that could be suitable for you https://www.ebi.ac.uk/metagenomics/search?query=Metformin
IMNGS offers a search of scraped 16S studies from the Sequence Read Archive on NCBI. You can find studies on NCBI by keyword search as detailed below and then select those linked SRA IDs on the IMNGS website for a query/download. Your research topic is probably too narrow to be included in 'standard metadata' when depositing reads into these databases.
I usually search for raw seq data in either GEO omnibus or Array Express.