As a drug to combat hair loss, the most common side effect is itchy scalp. Alcohol present in topical preparations may dry the scalp, resulting in dandruff. In some cases minoxidil may initially cause an increase in hair loss. This is called shedding and is seen as a long-term positive effect during hair loss treatment since the shedded hair strands will regrow within a few months with renewed strength.
There have been cases of allergic reactions to minoxidil or the non-active ingredient propylene glycol, which is found in some forms of topical Rogaine. Large amounts of minoxidil can cause hypotension, and it has been found that using petroleum jelly or tretinoin on the scalp with minoxidil can cause too much of the drug absorption by the scalp, as can using the drug on sunburned scalps.

If a person uses minoxidil to stop hair loss for a length of time and then stops taking the drug, hair loss will occur again.

Other side-effects include:

-Hyperprolactinemia, a disorder related to prolactin levels
-acne on the area where it is being used as a topical solution
-headaches and/or lightheadedness
-very low blood pressure
-irregular or fast heart beat
-blurred vision or other vision problems
-numbness or tingling in the hands, feet or face
-decreased sexual desire
-partial, or complete, impotence
-chest pain

All the side-effects in the above list except for acne may be an indicator that too much of the drug is being used, all of these side effects however are rare. It has also been found that the drug can be passed from a mother to a child via breast milk.