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: More serious ocular adverse reactions include papilledema , pseudotumor cerebri , and white or gray subepithelial corneal opacities ; all of these are reversible if the drug is discontinued .
### Output Text:
More serious ocular adverse reactions include papilledema , pseudotumor cerebri , and white or gray subepithelial corneal opacities ; all of these are reversible if the drug is discontinued . 