phototoxic

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Ancient Greekτόξον (tóxon)
9. Ancient Greekτοξῐκός (toxĭkós)
10. Ancient Greekτοξικόν (toxikón)
11. Latintoxicum
12. Latintoxicu
13. Frenchtoxique
14. Englishtoxic
15. English todayphototoxic

From photo- + toxic.