This discovery was completely by accident. After an initial surge of excitement with many people talking up the 'Miracle' in hair restoration. The hype died down and in many cases the results were disappointing.
It was then discovered that although minoxidil would most definitely re-grow hair -- it did nothing for the continued hair loss which was caused by hormones -- in both men and women.
Folks realized that for minoxidil to produce acceptable results -- using the solution would only partly solve the problem. What was needed was a way to stop continuous hair loss -- a way to control the hormones that caused hair to fall out -- and combining this second step with applications of minoxidil seemed to be the perfect solution.
Science discovered methods to reduce hormone levels, which slowed and even stopped continued hair loss. Using that and minoxidil together often produced outstanding results.
It is applied as a solution directly to the area where the hair is thinning and gently massaged in. Care must be taken that it doesn't come into contact with areas where you don't want hair growing.
But your original question still needs to be answered: Are there any minoxidil side effects from applying it to the scalp this way? Anything that would make using minoxidil dangerous or unsafe?
Minoxidil has been used for years by great numbers of people and there has never been a report of a dangerous or life threatening reaction to minoxidil, but as with all drugs care must be used when applying the solution.
Occasionally a small number of people will experience some minor skin irritation -- such as itching, flaking, dryness, and contact dermatitis (skin irritated by contact) and of course minoxidil is harsh enough that you need to avoid any accidental contact with eyes or other sensitive areas.
Never use any solution on broken or damaged skin, most do contain some alcohol, which will definitely cause discomfort.
These side effects are rare, minor and can be dealt with by washing the effected area with cold water.
The greatest side effect from minoxidil is its ability to re-grow hair.