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: For the subsequent 6 - week double - blind crossover phase ( phase B ) , patients taking standard - or low - dose haloperidol were switched to placebo , and patients taking placebo were randomly assigned to standard - or low - dose haloperidol .
### Output Text:
For the subsequent 6 - week double - blind crossover phase ( phase B ) , patients taking standard - or low - dose <span class="chemical">haloperidol</span> were switched to placebo , and patients taking placebo were randomly assigned to standard - or low - dose <span class="chemical">haloperidol</span> . 