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: METHODS : Review of all cases of corneal ulcers associated with drug abuse seen at our institution from July 2006 to December 2006 .
### Output Text:
METHODS : Review of all cases of corneal ulcers associated with drug abuse seen at our institution from July 2006 to December 2006 . 