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: Seven patients developed glucose tolerance curves characteristic of diabetes but these were mild , did not require treatment and returned to normal on ceasing didanosine .
### Output Text:
Seven patients developed glucose tolerance curves characteristic of diabetes but these were mild , did not require treatment and returned to normal on ceasing <span class="chemical">didanosine</span> . 