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 adverse reactions are frequent , less than 50 percent of patients are able to continue a particular drug for more than one year .
### Output Text:
Since adverse reactions are frequent , less than 50 percent of patients are able to continue a particular drug for more than one year . 