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: METHODS : A sample of 997 participants ( 52 % male ) was recruited to four control groups ( non - drug ( ND ) , alcohol / nicotine ( AN ) , cannabis / alcohol / nicotine ( CAN ) , non - ecstasy polydrug ( PD ) ) , and two ecstasy polydrug groups ( present ( MDMA ) and past users ( EX - MDMA ) .
### Output Text:
METHODS : A sample of 997 participants ( 52 % male ) was recruited to four control groups ( non - drug ( ND ) , <span class="chemical">alcohol</span> / <span class="chemical">nicotine</span> ( <span class="chemical">AN</span> ) , <span class="chemical">cannabis</span> / <span class="chemical">alcohol</span> / <span class="chemical">nicotine</span> ( <span class="chemical">CAN</span> ) , non - <span class="chemical">ecstasy</span> polydrug ( PD ) ) , and two <span class="chemical">ecstasy</span> polydrug groups ( present ( <span class="chemical">MDMA</span> ) and past users ( EX - <span class="chemical">MDMA</span> ) . 