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: Both are safe at high doses , but ranitidine is threefold more potent and does not cause the antiandrogen side effects frequently seen with high doses of cimetidine .
### Output Text:
Both are safe at high doses , but <span class="chemical">ranitidine</span> is threefold more potent and does not cause the antiandrogen side effects frequently seen with high doses of <span class="chemical">cimetidine</span> . 