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: Three patients received high doses of chlormethiazole for alcohol withdrawal symptoms , and one took a suicidal overdose of nitrazepam .
### Output Text:
Three patients received high doses of <span class="chemical">chlormethiazole</span> for <span class="chemical">alcohol</span> withdrawal symptoms , and one took a suicidal overdose of <span class="chemical">nitrazepam</span> . 