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: In the initial 6 months since it ' s introduction , 12 overdose cases have been reported to The National Poisons Information Centre .
### Output Text:
In the initial 6 months since it ' s introduction , 12 overdose cases have been reported to The National Poisons Information Centre . 