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: However , olanzapine - treated patients had a statistically significant greater mean ( + / - SD ) weight gain than placebo - treated patients ( 2 . 1 + / - 2 . 8 vs 0 . 45 + / - 2 . 3 kg , respectively ) and also experienced more treatment - emergent somnolence ( 21 patients [ 38 . 2 % ] vs 5 [ 8 . 3 % ] , respectively ) .
### Output Text:
However , <span class="chemical">olanzapine</span> - treated patients had a statistically significant greater mean ( + / - SD ) weight gain than placebo - treated patients ( 2 . 1 + / - 2 . 8 vs 0 . 45 + / - 2 . 3 kg , respectively ) and also experienced more treatment - emergent somnolence ( 21 patients [ 38 . 2 % ] vs 5 [ 8 . 3 % ] , respectively ) . 