If I use minoxidil 5% 2ml twice a day. Will the prescribed dose 1ml twice a day work effectively after that?

Yes it will, but you should only use the prescribed dose from now, on for the best results possible.

At 2 mL twice per day, you are exceeding the recommended FDA approved dose. By reducing the dose, hair loss can certainly occur. Speak to your physician about the appropriate steps now to balance safety and efficacy.

Using minoxidil is often trial and error. You need to find the right dose by experimentation and see what works for you