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: Ammonia ( NH3 ) was higher in patients who , during continuous therapy , complained of drowsiness ( 7 patients ) than in those who were symptom - free ( 17 patients ) , although VPA plasma levels were similar in both groups .
### Output Text:
<span class="chemical">Ammonia</span> ( <span class="chemical">NH3</span> ) was higher in patients who , during continuous therapy , complained of drowsiness ( 7 patients ) than in those who were symptom - free ( 17 patients ) , although <span class="chemical">VPA</span> plasma levels were similar in both groups . 