Sorry to take so long to reply, glad you got a whole bunch of good advice. Don’t panic, and follow your doctor’s advice. In a couple year’s time you will look back and laugh at yourself!
My step son has been on warfarin for 12 years with a mechanical mitral valve, and in a kid things fluctuate (diet, exercise, antibiotics, etc…) Going down to 1.8 is really not that big an issue. We self-dose and monitor. When he gets down to 1.8 for no particular reason that we can tell, we give him an extra 1-2mg immediately (his usual dose is 8.25mg, alternating 8 and 8.5 every other day). After that we continue his usual dose. We test again in 4 days. If his INR is back to normal, we maintain the dose and treat it as an anomaly. If it goes up a little bit, we maintain it and test again in another 4-5 days. Usually it stabilises. If the INR level is still low, it likely means that he has grown and his dose needs changing. In this case (since 1.8 is a small change), we would up his dose to 8.5mg daily for 4-5 days and see what happens.
It at times like this that I am so glad we self dose/monitor. Do keep in mind that depending on your metabolism warfarin is in your system for 3-7 days, so any dose change accumulates over 3 days…