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: The extrapolated 6 - hr serum concentration of cimetidine per unit dose , after intravenous cimetidine , increased with age of the patients .
### Output Text:
The extrapolated 6 - hr serum concentration of <span class="chemical">cimetidine</span> per unit dose , after intravenous <span class="chemical">cimetidine</span> , increased with age of the patients . 