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: The investigation was discontinued after 18 patients because of the frequency and intensity of side - effects , particularly pain and myoclonia , which caused the technique to be abandoned in two cases .
### Output Text:
The investigation was discontinued after 18 patients because of the frequency and intensity of side - effects , particularly pain and myoclonia , which caused the technique to be abandoned in two cases . 