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: Several drugs , including clonidine , physostigmine , methysergide , 5 - MDOT , propranolol , and MK - 801 , markedly reduced the dyskinetic movements but at the cost of a return of parkinsonian symptomatology .
### Output Text:
Several drugs , including <span class="chemical">clonidine</span> , <span class="chemical">physostigmine</span> , <span class="chemical">methysergide</span> , <span class="chemical">5 - MDOT</span> , <span class="chemical">propranolol</span> , and <span class="chemical">MK - 801</span> , markedly reduced the dyskinetic movements but at the cost of a return of parkinsonian symptomatology . 