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: Methamphetamine ( 10 mg / kg sc ) , administered five times , reduced the levels of dopamine and its metabolites in striatal tissue when measured 72 h after the last injection .
### Output Text:
<span class="chemical">Methamphetamine</span> ( 10 mg / kg sc ) , administered five times , reduced the levels of <span class="chemical">dopamine</span> and its metabolites in striatal tissue when measured 72 h after the last injection . 