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 determine the prevalence of heart disease in outpatient young asymptomatic chronic cocaine users , 35 cocaine users and 32 age - matched controls underwent resting and exercise electrocardiography ( ECG ) and Doppler echocardiography .
### Output Text:
To determine the prevalence of heart disease in outpatient young asymptomatic chronic <span class="chemical">cocaine</span> users , 35 <span class="chemical">cocaine</span> users and 32 age - matched controls underwent resting and exercise electrocardiography ( ECG ) and Doppler echocardiography . 