What is a synonym for "first-line"?

Common synonyms include: initial treatment.

First-line

adj

adj ·Rare ·Advanced level

Adjective

1
Of first resort. not-comparable
"The embankment was the first-line defense against the flood."
2
Of prime quality or importance. not-comparable
3
At the lowermost supervisory level. not-comparable
"first-line manager"

"Penicillin is a first-line treatment for bacterial infections, but doesn't work against viruses."