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: To address the potential confounding effects of the cannabis use of the ecstasy using group , a second analysis included 14 previously tested cannabis users ( Nestor , L . , Roberts , G . , Garavan , H . , Hester , R . , 2008 . Deficits in learning and memory : parahippocampal hyperactivity and frontocortical hypoactivity in cannabis users . Neuroimage 40 , 1328 - 1339 ) .
### Output Text:
To address the potential confounding effects of the <span class="chemical">cannabis</span> use of the <span class="chemical">ecstasy</span> using group , a second analysis included 14 previously tested <span class="chemical">cannabis</span> users ( Nestor , L . , Roberts , G . , Garavan , H . , Hester , R . , 2008 . Deficits in learning and memory : parahippocampal hyperactivity and frontocortical hypoactivity in <span class="chemical">cannabis</span> users . Neuroimage 40 , 1328 - 1339 ) . 