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: Although an infrequent complication of selective serotonin reuptake inhibitor therapy , it is important that ophthalmologists are aware that these agents can cause hypertension because this class of drugs is widely prescribed .
### Output Text:
Although an infrequent complication of selective <span class="chemical">serotonin</span> reuptake inhibitor therapy , it is important that ophthalmologists are aware that these agents can cause hypertension because this class of drugs is widely prescribed . 