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: Clinical experience with lovastatin includes over 5000 patients , 700 of whom have been treated for 2 years or more , and experience with simvastatin includes over 3500 patients , of whom 350 have been treated for 18 months or more .
### Output Text:
Clinical experience with <span class="chemical">lovastatin</span> includes over 5000 patients , 700 of whom have been treated for 2 years or more , and experience with <span class="chemical">simvastatin</span> includes over 3500 patients , of whom 350 have been treated for 18 months or more . 