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: Since it takes three to 12 months to achieve maximal effects , those patients who are unable to continue the drug receive little benefit from it .
### Output Text:
Since it takes three to 12 months to achieve maximal effects , those patients who are unable to continue the drug receive little benefit from it . 