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: Patients were treated with 1 tablet ( 500 mg of niacin extended - release / 20 mg of lovastatin ) once nightly for 4 weeks and then 2 tablets for 8 weeks .
### Output Text:
Patients were treated with 1 tablet ( 500 mg of <span class="chemical">niacin</span> extended - release / 20 mg of <span class="chemical">lovastatin</span> ) once nightly for 4 weeks and then 2 tablets for 8 weeks . 