Hello @riteshkundu. Oh goodness, I’m sorry your dad is feeling so poorly right now. You’re a wonderful daughter to be so concerned for his health and I know you’re very worried for him. The good news is that CML is usually quite treatable and patients may be stable for many years. Your dad was just recently diagnosed so it may take time to get it under control and balance the meds.

CML (chronic myeloid leukemia) is a type of cancer that begins in the bone marrow. It develops slowly in the blood-forming cells inside the marrow and eventually spreads through the blood. The good news is that treatments for CML can help slow the progression of the disease.
It sounds like your dad has had a rapid response to the meds in bringing down his white blood count, which should be encouraging.

Imatinib is a first line treatment for CML and has a good track record. Basically, it’s a medication that targets the gene that’s allowing too many white blood cells to be forming.
Here’s a good informational article for you that may help with the explanation: (there are several pages to the article)

From Mayo Clinic:
https://www.mayoclinic.org/diseases-conditions/chronic-myelogenous-leukemia/symptoms-causes/syc-20352417
It’s not uncommon with treatments such as Hydroxyurea for side effects to take place, especially with the very high dosage your dad was taking for 7 days. That’s because the body’s immune system is seriously compromised with the drop in white blood cells. Mouth sores, fatigue and low blood numbers are not unexpected. Once his neutrophil count returns to a more normal level, he should have signs of improvement in his mouth.
It’s also important to monitor your dad for any potential infections that you can’t see. That’s why the doctor gave instructions to check your dad’s temperature a couple times daily. If it reaches 101.4 you should call his doctor to get him admitted to the hospital for antibiotics.

I know this all sounds scary but with treatment your dad may have many years ahead of him. The meds will be important for his continued good health.
What does your father think is happening if he isn’t aware he has CML? Is there a reason you don’t want him to know?