import pandas as pd

df = pd.DataFrame({'person_id': [11,11,11],'text':['DOSE: 667 mg - TDS with food - Inject','DOSE: 667 mg - TDS with food - Tube','DOSE: 667 mg - TDS with food - PO'],'Summary':['Test','Test1','Test2']})
df['text'] = df['text'].str.replace(r'\bDOSE:\s*([^-]*)-[^-]*-\s*(.*)', r'\1\2')
print ( df.filter(['person_id', 'text', 'Summary']) )