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: Following a 3 - week observation period , volunteers were treated with zidovudine 600 mg / day for 18 weeks , 900 mg / day for 9 weeks and 1200 mg / day for 9 weeks , followed by a washout period of 6 weeks after which they were re - started on 1200 mg / day or the highest tolerated dose at 8 - hourly intervals .
### Output Text:
Following a 3 - week observation period , volunteers were treated with <span class="chemical">zidovudine</span> 600 mg / day for 18 weeks , 900 mg / day for 9 weeks and 1200 mg / day for 9 weeks , followed by a washout period of 6 weeks after which they were re - started on 1200 mg / day or the highest tolerated dose at 8 - hourly intervals . 