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: Treatment with high doses of cimetidine ( one to 60 months ; median , 11 months ) or ranitidine ( two to 31 months ; median , 14 months ) was not associated with hepatic or hematologic toxicity or alterations of serum gastrin concentrations , but ranitidine therapy was associated with a significantly lower serum creatinine level than seen with cimetidine therapy .
### Output Text:
Treatment with high doses of <span class="chemical">cimetidine</span> ( one to 60 months ; median , 11 months ) or <span class="chemical">ranitidine</span> ( two to 31 months ; median , 14 months ) was not associated with hepatic or hematologic toxicity or alterations of serum gastrin concentrations , but <span class="chemical">ranitidine</span> therapy was associated with a significantly lower serum <span class="chemical">creatinine</span> level than seen with <span class="chemical">cimetidine</span> therapy . 