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: Impotence was more common among male patients than controls and was found to be associated with co - morbidity and the taking of methotrexate .
### Output Text:
Impotence was more common among male patients than controls and was found to be associated with co - morbidity and the taking of <span class="chemical">methotrexate</span> . 