However, regular dosing is absolutely essential, and even missing one day leads immediately to death.
Literal
But regular [copula-な] dosage [subj-が] essential [copula-であり], one-day-[even-でも] not-dose [if-なければ] immediately death [to-に] reach.
であり is the conjunctive form of the formal copula である, used to chain clauses in written Japanese ('is X, and...'). たちまち 'immediately / all at once / in an instant' is a slightly literary adverb for sudden onset. ~に至る here applied to 死 'death' gives 'leads to death.' 服用 (formal 'taking of medicine') is medical vocabulary, paired with its verbalization 服用する. 一日でも ~なければ 'even one day not-X = if even one day of not-X.'