Does montelukast cause anosmia? didn't see it on possible side effects. happened suddenly after 14 days on this medication. negative for covid 19. no nasal congestion. no other medications. only symptom is smell loss?

Unlikely monteleukast causing loss of smell. Though you don't have nasal congestion, there could be inflammation/ local nerve damage/ nasal polyps etc. causing the symptoms. Usually it will clear up on its own if caused by allergies, cold. If no improvement, contact your PCP for further eval. of other causes of anosmia.

Doubtful that Montelukast caused your anosmia and consider COVID-19 PCR retest. Also evaluate nasal and sinus integrity with if necessary imaging of olfactory groove.