PubMed® by The National Center for Biotechnology Information, National Library of Medicine comprises more than 35 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full text content from PubMed Central and publisher web sites.

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Setup

You need to install a python package. 
pip install xmltodict

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Retriever

See a usage example. 
from langchain.retrievers import PubMedRetriever

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Document Loader

See a usage example. 
from langchain_community.document_loaders import PubMedLoader