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: Different mechanisms have been suggested for cocaine toxicity including an increase in oxidative stress but the association between oxidative status in the brain and cocaine induced - behaviour is poorly understood .
### Output Text:
Different mechanisms have been suggested for <span class="chemical">cocaine</span> toxicity including an increase in oxidative stress but the association between oxidative status in the brain and <span class="chemical">cocaine</span> induced - behaviour is poorly understood . 