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: These 6 patients had both renal and liver dysfunction ( P less than 0 . 05 ) , as well as cimetidine trough - concentrations of more than 1 . 25 microgram / ml ( P less than 0 . 05 ) .
### Output Text:
These 6 patients had both renal and liver dysfunction ( P less than 0 . 05 ) , as well as <span class="chemical">cimetidine</span> trough - concentrations of more than 1 . 25 microgram / ml ( P less than 0 . 05 ) . 