Most Serious: Heart issues such as rapid heartbeat (heart palpitations) and chest pain. If you experience this consult a doctor, we are NOT doctors. Minoxidil was originally developed for treating high blood pressure (hypertension).
Most Common: Itchiness, redness, and dryness. Minoxidil is alcohol-based, so make sure to hydrate your skin with lotion or beard oil. Many moisturizers and beard oils block DHT which will slow down your progress. That's why we recommend using this non-DHT blocking growth promoting beard oil.
Hair in Other Places: it’s possible that Minoxidil will promote hair growth in areas other than where applied (search sub for examples) like on ears, eyebrows, upper cheek, chest, stomach, hands, legs, back, etc. Most likely these will fall out once you stop Minoxidil use, as those hairs are still vellus and have not turned terminal. All you can do is: use topical Minoxidil as opposed to oral, only apply on areas you want gains, and avoid spreading the minoxidil to other areas (e.g. don’t apply right before bed and then sleep face-down).
Allergic to Minoxidil: If allergic to minoxidil, it is possible to replace propylene glycol with other vehicles such as butylene glycol, glycerin or polysorbate [1997].
Other: Rash/eczema (dermatitis), headaches, feeling faint/dizzy, increased weight, and fluid retention [2017].