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: These results show that curcumin has beneficial effect in mitigating the deterioration of cognitive functions and oxidative damage in rats treated with phenobarbitone and carbamazepine without significantly altering their serum concentrations .
### Output Text:
These results show that <span class="chemical">curcumin</span> has beneficial effect in mitigating the deterioration of cognitive functions and oxidative damage in rats treated with <span class="chemical">phenobarbitone</span> and <span class="chemical">carbamazepine</span> without significantly altering their serum concentrations . 