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: The effectiveness of haloperidol pretreatment in preventing the toxic effects of high doses of amphetamine and cocaine was studied in rats .
### Output Text:
The effectiveness of <span class="chemical">haloperidol</span> pretreatment in preventing the toxic effects of high doses of <span class="chemical">amphetamine</span> and <span class="chemical">cocaine</span> was studied in rats . 