PERNICIOUS.

Synonyms:

bad,evil,mischievous,pestilential,
baneful,foul,noisome,poisonous,
deadly,harmful,noxious,ruinous,
deleterious,hurtful,perverting,unhealthful,
destructive,injurious,pestiferous,unwholesome.
detrimental,insalubrious,

Pernicious (L. per, through, and neco, kill) signifies having the power of destroying or injuring, tending to hurt or kill. Pernicious is stronger than injurious; that which is injurious is capable of doing harm; that which is pernicious is likely to be destructive. Noxious (L. noceo, hurt) is a stronger word than noisome, as referring to that which is injurious or destructive. Noisome now always denotes that which is extremely disagreeable or disgusting, especially to the sense of smell; as, the noisome stench proclaimed the presence of noxious gases.

Antonyms:

advantageous,favorable,helpful,profitable,serviceable,
beneficent,good,invigorating,rejuvenating,useful,
beneficial,healthful,life-giving,salutary,wholesome.

PERPLEXITY.

Synonyms:

amazement,bewilderment,distraction,doubt,
astonishment,confusion,disturbance,embarrassment.