Know the red flags, have the right numbers to hand, and be able to say the right things in thirty seconds. Built to get you to help fast — never to tell you that help isn't needed.
Most divert overnight to a different provider entirely, and 2am is the worst possible time to discover that. It takes ninety seconds to find out and save it tonight.
What you actually need is the short list of signs that mean go now, learned in advance — plus the reminder that ringing an out-of-hours line costs nothing and they triage all day.
Weight, medications, when it started, what they ate and how much. It's exactly what the vet needs first and exactly what nobody can recall under stress — which is why it belongs on a card.
Which is dangerous with caustic substances, petroleum products, and in an animal that's already symptomatic. That call belongs to a vet or a poison control service who know the specific substance — never to a website.