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: While there is no single correct starting dose for levodopa therapy , many individuals can be started on either the 25 / 100 or controlled - release formula , following the general rule not to attempt to titrate carbidopa - levodopa to the point of " normality , " which can lead to toxicity .
### Output Text:
While there is no single correct starting dose for <span class="chemical">levodopa</span> therapy , many individuals can be started on either the 25 / 100 or controlled - release formula , following the general rule not to attempt to titrate <span class="chemical">carbidopa</span> - <span class="chemical">levodopa</span> to the point of " normality , " which can lead to toxicity . 