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: Symptomatic adverse effects were present in 96 % of subjects , most commonly nausea ( 64 % ) , fatigue ( 55 % ) and headache ( 49 % ) .
### Output Text:
Symptomatic adverse effects were present in 96 % of subjects , most commonly nausea ( 64 % ) , fatigue ( 55 % ) and headache ( 49 % ) . 