Is it safe to use Minoxidil 5% on healthy hair?

Minoxidil is effective when it is in use. When you quit you usually loose the hair gained through minoxidil use. It is better to consult with your doctor before using it.

Minoxidil is a FDA-approved topical medication for hair growth that works well for androgen (hormone) related hair loss.  It helps extend the growth cycle of hair and
decrease shedding that naturally occurs. 
For minoxidil to work, there is an activation process.  During the first four weeks, fine hairs may
shed where thicker hairs may replace. 
During this time, it may appear you are losing hair when it is actually
working.  However, if you stop using
minoxidil, your hair will revert back to the same growth cycle.  I recommend a consultation with an expert
in hair restoration. 

It is normal to shed hair every day.  It is estimated that you will loose 100 hair each day normally.  You could try counting the hair you are shedding.  There is another test that will tell you if you are starting to thin.  This is a cross sectional trichometry comparing the back of your head with the top of your head.  If both numbers are similar, you are not likely thinning and you will not benefit from minoxidil.  There are some considerations however.  All hair in all men and women begins to age with time beginning at age 35.  Things like Wnt-act, and Haircycle products can help you maintain a more youthful microenviroment for you hair and help keep the hair in place over time.  If you are loosing your hair, you might consider medications like minoxidil, but you first need to understand if you are naturally aging or whether you have androgenic alopecia.