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: However , ranitidine was threefold more potent than cimetidine both in acute inhibition studies and in the median maintenance dose needed ( 1 . 2 g per day for ranitidine and 3 . 6 g per day for cimetidine ) .
### Output Text:
However , <span class="chemical">ranitidine</span> was threefold more potent than <span class="chemical">cimetidine</span> both in acute inhibition studies and in the median maintenance dose needed ( 1 . 2 g per day for <span class="chemical">ranitidine</span> and 3 . 6 g per day for <span class="chemical">cimetidine</span> ) . 