noun, masculine/filtʁ/plural: philtres
philtre is masculine in French.
philter (a kind of potion, charm, or drug intended to make the drinker fall in love with the giver)
“Dans quel philtre, dans quel vin, dans quelle tisane, / Noierons-nous ce vieil ennemi, / Destructeur et gourmand comme la courtisane, / Patient comme la fourmi?” — What wine, what drug, what philtre known of man / Will drown this ancient foe, / Ruthless and ravenous as a courtesan, / Sure as an ant, and slow?
Borrowed from Latin philtrum, itself a borrowing from Ancient Greek φίλτρον (phíltron). Unrelated to filtre.