The book The Romance of Tristen and Iseult was filled to the brim with symbolism, the sword, the dragon, the philter, and plenty of others. Each of these has many paradoxical, similar, Ironic, oxymoronic, and contradictory symbols. The Philter, in particular, symbolizes several contradictory ideas.
The Philtre was a potion that made both drinkers of the potion fall in love. There must be some rational explanation for why Tristen and Iseult went from enemies to lovers. One very possible answer is that the potion was a very potent aphrodisiac. Meaning they didnt fall in love but in lust. The queen must have wanted Iseult and the king to drink that and have a very productive honeymoon. Also by giving this potion to Iseult she would be much more willing to go to bed with this stranger that she will be marrying. Tristen and Iseult drank this potion and from then on they were screwing at every possible moment, as this quote from the book shows. But in every hour and place every man could see Love terrible, that rode them, and could see in these lovers their every sense overflowing like new wine working in a vat. Going from one hating the other and the other bringing her home for his beloved king to making love anywhere they can would have to be a very powerful sexual stimulant.
The contradictory representation of the Philtre would be Love. Once drinking this potion the drinkers were supposed to fall madly in love for all time. The two who drink from this would never waver from one another. As proven in this passage when Ogrin was speaking to Tristen.
The King has published a ban in every parish: Whosoever may seize you shall receive a hundred marks of gold for his guerdon, and all the barons have sworn to give you up alive or dead. Do penance, Tristen! God pardons the sinner who turns to repentance
And what should I repent, Ogrin, my lord? Or of what crime? You that sit in judgment upon us here, do you know what cup it was we drank upon the high sea? That good, great draught inebriates us both. I would rather beg for my life long and live of roots and herbs with Iseult than lacking her, be king of a wide kingdom.
Tristen was willing to throw away an entire kingdom to be with Iseult forever. Even if it meant that they would have to survive on plants and scraps of food they could find in the forest.
These symbols still have some relevance in the modern world. Two people do fall in love over a deep drink though not of a magic potion, but a high proof alcoholic beverage. Two people drinking heavily of whisky or beer will not fall instantly in love forever. Though those to drinkers will fall instantly in love until they roll over in the morning in a strange bed and scream in horror when they see the person/thing/hideously wretched abomination the slept with the night before. The ideals of a magic potion making people fall in love are still alive in the minds of adolescents who have fallen madly in love with some other student or one of their teachers and are too shy to try the direct approach.