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: All participants received 1 . 0 mg or 2 . 0 mg trihexyphenidyl or placebo administered in counterbalanced sequences over a period of three consecutive weeks .
### Output Text:
All participants received 1 . 0 mg or 2 . 0 mg <span class="chemical">trihexyphenidyl</span> or placebo administered in counterbalanced sequences over a period of three consecutive weeks . 