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: In patients with downbeat nystagmus who are taking anticonvulsant medications , consideration should be given to reduction in dose before further investigation is undertaken .
### Output Text:
In patients with downbeat nystagmus who are taking anticonvulsant medications , consideration should be given to reduction in dose before further investigation is undertaken . 