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: There was only one case of dementia possibly due to cimetidine ( with a drug level of 1 . 9 microgram / ml 6 hr after a dose ) in a group of 13 patients without liver or kidney disease who had cimetidine levels above 1 . 25 microgram / ml .
### Output Text:
There was only one case of dementia possibly due to <span class="chemical">cimetidine</span> ( with a drug level of 1 . 9 microgram / ml 6 hr after a dose ) in a group of 13 patients without liver or kidney disease who had <span class="chemical">cimetidine</span> levels above 1 . 25 microgram / ml . 