| Fr1 | first fraction |
|---|---|
| FRS | Fellow of the Royal Society; ferredoxin-reducing substance; first rank symptom; furosemide |
| FUS | feline urologic syndrome; first-use syndrome |
| L1, | L2, L3, L4, L5 first, second, third, fourth, and fifth lumbar vertebrae |
| LI, LII, | LIII first, second, third stage of syphilis |