HAIR LOSS
What can weaken minoxidil, and how to use it safely
In short
Minoxidil needs a scalp enzyme to work, and aspirin lowered it in a small study. The best-proven hair-loss options are minoxidil and anti-DHT drugs such as finasteride.
What you need
Do this
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1
Minoxidil must be switched on by the enzyme SULT1A1 in your follicles.
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2
Daily low-dose aspirin lowered predicted response.Why
Ask a doctor before changing any aspirin.
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3
Tretinoin raised the enzyme in a small study.Why
Talk to a doctor first.
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4
What the science points to: similar pain pills may too (possible).
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5
Keep cats away from the product, your wet scalp and your pillowcase.
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6
Itchy scalp? Foam has no propylene glycol.Why
A patch test tells allergy from irritation.
The numbers behind it
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50% to 27%
Share of 24 people predicted to respond to minoxidil before and after 14 days of daily low-dose aspirin. Hair growth was not measured.
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43%
Of people predicted not to respond to minoxidil were predicted to respond after 5 days of topical tretinoin.
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75% vs 33%
Regrew hair with 5% minoxidil plus an enzyme booster versus a sham booster after 60 days (24 men).
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8 of 62
Cats with signs after exposure to an owner's minoxidil died (211 dog and cat cases from a poison centre).
For your hair
Pain pills (liver-tissue data)
Mefenamic acid and salicylic acid blocked the same enzyme in human liver tissue. What the science points to: a weaker minoxidil effect is possible.
Curcumin and quercetin tablets (lab-tissue data)
Both block the enzyme in lab tissue. What the science points to: tablets may not reach follicles, so a weaker effect is less likely.
Cat and dog owners
Cats were mostly exposed by licking an owner's skin or pillowcase. Wash your hands after applying and let it dry before bed.
Daily aspirin users
Aspirin is usually taken for the heart. Never stop it on your own. Talk to a doctor about whether this matters.
Myths to ignore
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Myth Itch from minoxidil is always the propylene glycol in the liquid.
Truth In 99 patch-test-confirmed allergy cases, minoxidil itself was the allergen 4 times as often as propylene glycol. A patch test separates allergy from irritation.
Kiratiwongwan 2025 ↗ -
Myth Itchy and red after minoxidil always means allergy.
Truth Irritation looks the same as allergy. People sensitive to propylene glycol can be offered other formulations such as foam.
Junge 2025 ↗
Go deeper
- Minoxidil forms comparedSolution, foam, oral tablets, sublingual and extended-release minoxidil deliver the same drug in different ways; this page compares them.
- Hair growth pipeline (2026)What is in clinical trials for hair loss as of October 2026, how far along it is, and how much to trust the early numbers.
- DHT and pattern hair lossPattern hair loss happens when genetically sensitive scalp follicles respond to the hormone DHT by shrinking a little more with every hair cycle.
- Androgenetic alopecia (male and female pattern hair loss)The most common cause of hair loss is an inherited, slowly progressive shrinking of scalp follicles; it cannot be cured, but finasteride, minoxidil and transplants slow, partly reverse or camouflage it.
- When to see a dermatologistMost gradual thinning is pattern loss you can start treating yourself, but scarring signs, scalp symptoms, patches or sudden shedding should go to a clinician quickly because delay can cost permanent hair.
All studies in this guide (9)
Goren A et al. Low-dose daily aspirin reduces topical minoxidil efficacy in androgenetic alopecia patients. Dermatol Ther. 2018
Tested: low-dose oral aspirin (75-81 mg/day) with hair follicle sulfotransferase enzyme activity measured before and after
Result: Half of the 24 people (50%) were predicted to respond to topical minoxidil at the start. After 14 days of low-dose aspirin only 27% were. Hair growth itself was not measured.
Read the study ↗Sharma A et al. Tretinoin enhances minoxidil response in androgenetic alopecia patients by upregulating follicular sulfotransferase enzymes. Dermatol Ther. 2019
Tested: topical tretinoin for 5 days
Result: Tretinoin raised follicular sulfotransferase and 43% of predicted non-responders were converted; the group is tied to a commercial response test.
Read the study ↗Dhurat R et al. SULT1A1 (minoxidil sulfotransferase) enzyme booster significantly improves response to topical minoxidil for hair regrowth. J Cosmet Dermatol. 2022
Tested: 5% minoxidil daily plus a topical enzyme booster versus plus a sham booster
Result: 75% of men using the booster regrew hair versus 33% using the sham (12 per group, judged on global photos).
Read the study ↗Pacifici GM. Inhibition of human liver and duodenum sulfotransferases by drugs and dietary chemicals: a review of the literature. Int J Clin Pharmacol Ther. 2004
Tested: mefenamic acid, salicylic acid and quercetin tested against sulfotransferase activity
Result: Mefenamic acid, salicylic acid and quercetin all blocked the enzyme SULT1A1 in human liver tissue, and quercetin blocked the sulfation of minoxidil. Nothing was tested in hair follicles.
Read the study ↗Vietri M et al. Curcumin is a potent inhibitor of phenol sulfotransferase (SULT1A1) in human liver and extrahepatic tissues. Xenobiotica. 2003
Tested: curcumin added to tissue samples
Result: Curcumin blocked SULT1A1 strongly in every tissue at nanomolar levels, far below the level reached in blood after a 4 g dose. Hair follicles were not tested.
Read the study ↗Tater KC et al. Topical minoxidil exposures and toxicoses in dogs and cats: 211 cases (2001-2019). J Am Anim Hosp Assoc. 2021
Tested: accidental exposure to topical minoxidil
Result: Signs followed even a few drops or licks; 8 of 62 symptomatic cats died.
Read the study ↗Kiratiwongwan R et al. Allergic contact dermatitis to topical preparations containing minoxidil: a systematic review and individual participant data meta-analysis. Dermatitis. 2025
Result: Minoxidil itself was the allergen in 74.7% and propylene glycol in 17.1%.
Read the study ↗Junge A et al. Contact dermatitis caused by topical minoxidil: allergy or just irritation. Acta Derm Venereol. 2025
Result: Among patch-test-positive people, 54 reacted to minoxidil itself and 12 to the other ingredients in the product. Redness, itch and scaling after use need a patch test to tell allergy from irritation, and people sensitive to propylene glycol can be offered other formulations.
Read the study ↗Olsen EA et al. A multicenter, randomized, placebo-controlled, double-blind clinical trial of a novel formulation of 5% minoxidil topical foam versus placebo in the treatment of androgenetic alopecia in men. J Am Acad Dermatol. 2007
Tested: 5% minoxidil foam (propylene-glycol-free) vs placebo
Result: Hair counts rose significantly more than with placebo (P<0.0001) and the foam was well tolerated for up to 52 weeks.
Read the study ↗