The majority of benefit comes from 2.5mg, but 5mg is proven to give better results. There’s no additional benefit with doses higher than 5mg.
I have a prescription for 5mg / day, but in the US there are no 5mg tabs, so I get 2x2.5mg.
The thing is when I take both, I get some water retention in my face, which doesn’t look good on me. This is highly individual, so probably won’t impact most people.
By splitting the 5mg dose up into morning and evening I avoid the water retention.
So if you’re just starting, I suggest taking 2.5mg for a month, to allow your body to adjust. If there’s any water retention it usually resolves in that time. Then increase to 5mg for a month. Again, any water retention, if it happens, should resolve after a month. If not, split into two (2.5mg) doses a day.
If that’s too much to deal with, 2.5mg once a day will still work well. Unlike topical, which can be completely ineffective for people lacking a certain enzyme in the scalp needed to convert minoxidil to.the active form, oral is 100% effective. The liver has an abundant supply of the enzyme so oral is completely converted in everyone.