There are 8 13-letter words that start with 'eff'.
| effectiveness | effectivities | 
| effectualness | effectuations | 
| effervescence | efficaciously | 
| efflorescence | effortfulness | 
Note: These 8 13-letter words that start with 'eff' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with effe, effl, effu, effi, effa, effo, effr, effs.
4-letter words start with 'eff' (1)