Although minoxidil is generally well tolerated, it is contraindicated in some specific cases because minoxidil can interact with certain medications. More so for the oral version rather than the topical version.
Let me explain who should not use minoxidil and why.
Topical minoxidil contraindications
Possible contraindications against using topical minoxidil are:
Women who are pregnant
It is unsure if minoxidil is harmful to the unborn child since it doesn’t seem to impact fertility. However, there’s very little systematic absorption when minoxidil is applied topically.
Oral minoxidil will have a much higher systematic absorption rate and thus, it can be assumed to be potentially much more harmful to the unborn, still developing child.
Personally, if possible, I wouldn’t advise using it during pregnancy. Better to be safe than sorry. If you are going to use it anyway, then the topical formula is your safest bet.
Women who are breastfeeding their children
Again, since systematic absorption of minoxidil is a thing, much more so for oral minoxidil than topical minoxidil, it is potentially dangerous when breastfeeding a baby because it is possible to pass it on to the infant.
Adults are fully developed both physically and emotionally and are larger, and thus, can generally handle larger doses of minoxidil without problems. With little children, who are still in development and much smaller, we don’t know how much minoxidil they can handle, or if they can handle any trace of the drug at all.
Thus, just to be safe, women are best off to stop minoxidil during the full-time period that they are breastfeeding. Although the risks of topical minoxidil will in this regard be much smaller than with oral minoxidil due to, again, less systematic absorption in the case of topical minoxidil.
People with a history of hypersensitivity to minoxidil or certain components
Some people are hypersensitive to certain elements of minoxidil or component that are present in minoxidil such as, for example, propylene glycol[1]https://www.ncbi.nlm.nih.gov/books/NBK482378/
Systemic minoxidil, also commonly called oral minoxidil, is a potent vasodilator with the potential to produce hypotension and reflex tachycardia. In some rare cases, serious complications may occur.
Oral minoxidil contraindications
Oral minoxidil is also relatively contraindicated in patients who are pregnant, are breastfeeding and/or have a hypersensitivity to minoxidil or certain components and who are suffering from:
Cardiac disease (including angina, coronary artery disease, recent or acute myocardial infarction)
Since oral minoxidil was historically used to treat hypertension, it lowers the blood pressure. Topical minoxidil has not been shown to do so due to less systematic absorption. In some cases, minoxidil might cause a faster heartbeat, which may lead to myocardial problems such as erythema in some cases.
Cerebrovascular diseases
In uncommon cases, a reflex increase in heart rate and decrease in blood pressure can worsen these already pre-existing conditions.
Patients with coronary insufficiency to avoid the risk of reflex tachycardia and angina exacerbation
Minoxidil may cause pericardial effusion, which occasionally may progress to cardiac tamponade. Tachycardia as a result of reflex sympathetic activation may account for ECG changes.[2]https://www.sciencedirect.com/topics/medicine-and-dentistry/minoxidil
Oral minoxidil should be reserved for patients suffering from hypertension who do not respond adequately to peak therapeutic doses of a diuretic
In experiments with animals, minoxidil has been shown to potentially induce several types of myocardial lesions as well as other adverse cardiac effects. Although the evidence is certainly not conclusive and more studies need to be done to confirm these suspicions.[3]https://www.pdr.net/drug-summary/Minoxidil-minoxidil-774
Conclusion
As we can see, in some specific cases and rare instances, there can be contraindications of minoxidil use. In particular oral minoxidil contraindications, since there is more systematic absorption with oral use than is the case with topical minoxidil.
That’s not to say that the possible negative effects of minoxidil are necessarily harmful or strongly scientifically validated to be dangerous. But, it is possible that minoxidil can have dangerous side effects, and thus, minoxidil contraindications include those people with certain pre-existing conditions and risk profiles.
We don’t know if minoxidil can be unsafe for an unborn child, or if it is harmful to a small child when breastfeeding. But, in those cases, I would advise caution. And it is better to be safe than sorry in my humble opinion.