Short answer: Best to avoid it.

Reason:

• Paryushan is a time for maximum samyam (self‑restraint) and ahimsa. Fermented foods like curd and buttermilk contain many subtle living beings (nigodas). Avoiding them reduces himsa and supports inner purity.

Tradition notes:

• Shvetambar practice: During the 8 days, households commonly give up curd, buttermilk, and other fermented items, along with stricter rules like no green vegetables and boiled water only in daytime.
• Digambar practice (Das Lakshan): Observers keep strong vrats such as upvas/ekasana and avoid fermented foods; buttermilk is generally not taken.

If health truly requires:

• Traditional leniency is to prefer non‑fermented, simple items (e.g., plain boiled milk) taken only in daylight. If buttermilk is unavoidable, it should be freshly prepared the same day, not sour, and only during daylight—but the ideal remains to renounce it for the festival period.