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: Although visual hallucinations have not been reported as an adverse effect of this agent , we describe three patients who experienced complex visual hallucinations and altered mental status after zonisamide treatment was begun or its dosage increased .
### Output Text:
Although visual hallucinations have not been reported as an adverse effect of this agent , we describe three patients who experienced complex visual hallucinations and altered mental status after <span class="chemical">zonisamide</span> treatment was begun or its dosage increased . 