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: Among women with cardiovascular disease , long - term use of combined continuous HT significantly increased the risk of venous thrombo - embolism . One trial analysed subgroups of 2839 relatively healthy 50 to 59 year old women taking combined continuous HT and 1637 taking oestrogen - only HT , versus similar - sized placebo groups .
### Output Text:
Among women with cardiovascular disease , long - term use of combined continuous HT significantly increased the risk of venous thrombo - embolism . One trial analysed subgroups of 2839 relatively healthy 50 to 59 year old women taking combined continuous HT and 1637 taking <span class="chemical">oestrogen</span> - only HT , versus similar - sized placebo groups . 