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: After the ingestion of 26 venlafaxine 50 - mg tablets , the patient experienced a witnessed generalized seizure .
### Output Text:
After the ingestion of 26 <span class="chemical">venlafaxine</span> 50 - mg tablets , the patient experienced a witnessed generalized seizure . 