The preferred MeSH term for opioid addiction is Opioid-Related Disorders. The following are other terms that MeSH automatically maps to the term Opioid-Related Disorders:
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These are called entry terms and are helpful for researchers because MeSH will automatically connect the entry terms to the selected MeSH term. Notice that Opioid Addiction is an entry term for Opioid-Related Disorders. If you search PubMed with Opioid Addiction, PubMed will also retrieve results indexed with Opioid-Related Disorders.
Entry terms (similar to the idea of "cross-references"), are synonyms, alternate forms, and other closely related terms in a given MeSH record that are generally used interchangeably with the preferred descriptor term for the purposes of indexing and retrieval. This greatly increases the access points to the citations since a searcher may not know the exact preferred term for a main heading and there are frequently terms which mean the same thing when the article is indexed.