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: However , METH significantly increased the immobility time in the tail suspension test at 3 and 49 days post - administration .
### Output Text:
However , <span class="chemical">METH</span> significantly increased the immobility time in the tail suspension test at 3 and 49 days post - administration . 