Viagra (sildenafil) can reduce ability to smell.

  Recent research suggests that Viagra may cause a reduced sense of smell in some patients.

  This might be secondary to nasal congestion...reported to occur in about 20% of Viagra users. Nasal congestion has also been reported with Cialis and is likely a class effect. It's thought that PDE-5 inhibition enhances the vasodilatory actions of nitrous oxide in the nasal mucosa leading to nasal obstruction.

  Reassure patients that any nasal congestion or smell disturbances are likely to disappear once the drug is stopped. For patients who take a PDE-5 inhibitor chronically and can't tolerate it because of nasal congestion or smell problems, suggest lowering the dose if possible.