No. It is fine if you correct the spelling of "previously".
Student or Learner
'The results indicate that the new drug is safer than was previeuosly assumed.'
Doesn't 'was' need a subject?
“Time stays long enough for anyone who will use it.”
- Leonardo da Vinci
You can omit "was" and it will still be an acceptable sentence.
The results indicate that the drug is safer than previously assumed.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.