In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Although clinical experience with omeprazole is still limited , many controlled studies have established the short - term safety of this drug .
### Output Text:
Although clinical experience with <span class="chemical">omeprazole</span> is still limited , many controlled studies have established the short - term safety of this drug . 