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: Emergency physicians should be aware of oral penicillin anaphylaxis in order to prevent its occurrence by prescribing the antibiotic judiciously and knowledgeably and to offer optimal medical therapy once this life - threatening reaction has begun .
### Output Text:
Emergency physicians should be aware of oral <span class="chemical">penicillin</span> anaphylaxis in order to prevent its occurrence by prescribing the antibiotic judiciously and knowledgeably and to offer optimal medical therapy once this life - threatening reaction has begun . 