Procedure

Cantharidin

A liquid made from a blister beetle extract, painted onto molluscum bumps or warts in the office and washed off at home at a set time. It causes a small, controlled blister that lifts the lesion off. Putting it on does not hurt, which is why it suits young children. The blister that follows can.
Dr. Schwarz, MD · Board-certified dermatologistLast reviewed September 8, 2026
16:9 hero for Cantharidin. Never cropped: the tone strip and the corner logo depend on the full frame.

Start here

Cantharidin is what I reach for first in a young child with molluscum who needs treating at all. Nothing is frozen, nothing is scraped, and the child feels nothing while it is being painted on. The wash-off time is an instruction, not a suggestion — left on longer, the blister is bigger and more painful. Molluscum also clears on its own, so treating it is a choice, usually made because bumps are spreading, itchy, sore, or keeping a child out of activities.

— Dr. Schwarz, Board Certified Dermatologist

Key Facts

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Also calledBeetle juice, blister beetle extract, cantharone. The FDA-approved version for molluscum is sold as Ycanth
Used forMolluscum contagiosum, and common warts including warts around the nails
Applied byA provider in the office, with a wooden applicator, to the bumps only. It is never dispensed for you to apply at home
Washed offBy you, at home, at the exact time you are given. This is the safety step
DowntimeNone from the visit itself. A blister forms within hours and heals over about 1 to 2 weeks
SessionsRepeated every 2 to 4 weeks until lesions clear, often 2 to 4 visits for molluscum and more for warts
PainNone while it is applied. The blister that follows can be sore or itchy for a day or two
Typical costCompounded cantharidin is inexpensive and most of the charge is the office visit. The branded FDA-approved product costs considerably more and coverage varies. Ask which is being used and what it will cost
Skin tone safetySuitable for all skin tones. A temporary dark or pale mark at each treated spot is common on deeper skin tones and usually fades over weeks to months
Not used onThe face, the eyelids, the eyes, and mucous membranes such as the lips or genital skin

What It Is

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Cantharidin is a chemical made by blister beetles, the group that includes the insect known as Spanish fly. It is purified into a topical solution, usually around 0.7 percent. It is an in-office procedure, not a prescription you take home. A provider paints a small amount onto each bump, lets it dry, and sends you home to wash it off at a set time. You never handle the liquid — contact time controls the size of the blister, and the liquid is poisonous if swallowed. For decades cantharidin was compounded by pharmacies and used off-label, with no standard strength. In 2023 an FDA-approved solution for molluscum contagiosum in patients aged 2 and over became available in the US, sold as Ycanth. Both are still used, with different contact times, so the instruction you are given matters more than anything you read in general.

How It Works

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Cantharidin soaks into the outer layer of the skin and triggers enzymes that break down the connections holding skin cells together. Those connections are desmosomes, and without them the cells come apart.

The skin splits along that layer and fluid fills the gap, which is the blister. Where the split happens matters — inside the outer layer, above the base layer that regenerates skin. Because the rebuilding layer is intact, a cantharidin blister usually heals without a scar.

A molluscum bump sits in that outer layer, so when the roof of the blister lifts, the infected skin comes away with it. Cantharidin does not kill the virus or act on the immune system, though the local inflammation may help the body notice an infection it had been ignoring.

Warts are harder to clear: They sit deeper and are tougher, so they respond less reliably and often need occlusion, repeat treatments, or a different approach.

Skin is about two millimetres thick on the face. How deep a treatment reaches decides what it can change, and whether it treats all of the skin or scattered columns of it decides how long you take to heal.

How it works
Compare
Skin basics
EPIDERMISDERMISFAT0.05 mm — pigment0.5 mm — texture + pores1.0 mm — collagen1.5 mm — deep dermisPEELNEEDLINGNON-ABLATIVE LASERABLATIVE LASERSTAYS IN THE TOP LAYER

This works in the epidermis, the top layer, only about a tenth of a millimetre thick. That is the right depth for surface pigment, flaking and rough texture, and it is why nothing at this depth can change a scar or a wrinkle that is set in the layer below.

A peel or a scrub takes the skin off evenly from the surface down. Everything above the line goes and nothing below it is touched, which is why peels suit surface pigment and rough texture, and why the whole face flakes for a few days after.

Needles make hundreds of separate channels and leave the skin between them untouched. That untouched skin is what heals the rest, so recovery is quick — but a needle only injures the skin to start repair. It removes nothing.

The light passes through the surface and heats a column underneath it. The surface stays whole, so you leave red rather than raw — and it takes a course of sessions rather than one.

The laser removes columns of skin outright: the white gaps here are tissue that is gone, and new skin grows in from the sides. The strongest of the four, and the longest to heal.

The outer layer, and the only one anything in a jar reaches. It renews itself constantly: a cell made at the bottom takes about a month to reach the surface and flake off. Most of what a skincare product does, it does here.

The living layer underneath, holding the blood vessels, the nerves and the collagen. It is where lasting change happens and it is hard to reach: most of what is sold for the skin never gets this far.

What It Treats

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Molluscum Contagiosum
Molluscum Contagiosum
Moderate evidence
A blistering liquid painted onto each bump in the clinic and washed off a few hours later at home. A small blister forms over the next day and the bump lifts off with it. It is painless to apply, which is why it suits children, and it usually takes two or three visits. It is not used on the face or the genitals, and it can leave a dark or pale mark, so a few bumps are treated first to see how the skin reacts.
Warts
Warts
Moderate evidence
A blistering liquid painted on in clinic and washed off a few hours later. It is painless going on, which is why it is a common first choice for children and for warts on the face. The blister that follows lifts the infected skin away over the next few days and can be sore. It usually takes more than one visit.

Not the Best For

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Plantar Warts
Plantar Warts
Limited evidence
A blistering liquid painted on in clinic and washed off a few hours later, which lifts the wart away from the skin beneath. It is painless to apply, which makes it a common choice for children. The blister that follows can be sore to walk on for a few days, and on the thick skin of the sole it often needs repeating.
New Bumps Appearing
Doesn't work
Cantharidin only removes the bumps it is painted on. Molluscum keeps producing new bumps until the immune system recognizes the virus, which can take months. Repeat visits every 2 to 4 weeks are normal and are not a sign the treatment failed.

Pros and Cons

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Pros
  • Painless to apply No needles, freezing, scraping or numbing cream.
  • The visit is quick A handful of lesions takes a few minutes.
  • Usually heals without scarring The blister forms above the regenerating layer.
  • Works in awkward places Around the nails, where freezing is especially painful.
  • No home handling of medicine The provider applies it, so nobody overdoes it.
Cons
  • The blister is the treatment Painless in the office, sore hours later at home.
  • Timing is on you Wash off at the hour you were given. Set an alarm.
  • Several visits Molluscum needs repeating every few weeks until it burns out.
  • Ring lesions New bumps or a ring of warts can appear around a treated spot.
  • Limited high-quality evidence Long track record, thin trial data.
  • Not for every site The face, eyelids and genital skin are off limits.

How to Prepare

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A few days beforeOn the day
Actives
A few days before
Skip salicylic acid, retinoids and strong exfoliants on the area. They thin the surface and make the blister bigger than intended.
Clean skin
On the day
Clean, dry skin. Point out the bumps that bother you or your child most, since providers often treat a limited number first.
Before you book
Work out when the wash-off falls and whether you will be near soap and water. A morning appointment is easier. Wear loose clothes.

What Happens

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ArrivingThe treatmentLeaving
Arriving
The lesions are counted and the provider decides how many to treat today. You are told which product is used and how long it stays on. Nothing sharp touches the skin, so no numbing is needed. Your provider will ask about these things.
The treatment
Liquid is dabbed onto each bump with a wooden applicator, touching only the lesion, and air dries for a minute or two. Some providers tape a wart afterward, which makes it stronger and changes the contact time, so ask. Several lesions take 5 to 15 minutes. You feel nothing during the visit; the discomfort comes hours later as the blister forms.
Leaving
You go home with the liquid still on the skin. Keep the area uncovered or loosely covered, stop a child touching it and rubbing their eyes or mouth, and set an alarm.

Recovery

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First hoursHours 4 to 24Weeks 1 to 2
First hours
The spots look unchanged and you feel nothing. Keep the area dry. At the exact hour you were given, wash with soap and water, rub gently, and dry it. Longer does not work better.
Hours 4 to 24
A clear or cloudy blister rises over each treated bump. This is the treatment working, not a reaction to it. It may itch, sting or feel sore. Over days 2 to 7 it flattens, drains, and the roof dries into a crust.
Weeks 1 to 2
The crust separates and takes the lesion with it, leaving pink, brown or pale new skin. Remaining and new lesions are treated 2 to 4 weeks later.

Aftercare

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At the set hourFrom the next dayOnce dry and crusted
Washing off
At the set hour
Soap, water and gentle rubbing at the exact hour you were given. Set an alarm.
Bathing
From the next day
Normal after the wash-off. Pat dry rather than rubbing.
Swimming and sports
Once dry and crusted
Fine once blisters have dried and crusted. Keep intact blisters covered.
The blister
Leave it alone. If one is large and painful, call the office rather than opening it.
Covering
A loose bandage stops rubbing and stops a child picking.
Sun
Protect healing spots. Sun on a fresh mark makes it darker and slower to fade.
Hands
Wash hands after touching the area, and keep treated skin away from eyes and mouth.
Picking
Do not pull crusts off early. Picking is the main way these sites end up with a lasting mark.

Risks

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The blister is intended, not a complication. Blisters that are bigger, more painful or in the wrong place than planned nearly always trace back to contact time.

A blister much larger than expected
Especially one that is tense, painful or crosses a joint crease. Reduce the contact time or the number of lesions next visit.
Pain keeping a child awake
Worth a call. Painkillers and a dressing usually settle it, and the plan can be adjusted.
A ring of new bumps around a treated spot
Sometimes called ring warts. Harmless but frustrating, and a reason to change approach.
A lasting dark or pale patch
Common on deeper skin tones, usually temporary, and easier to treat early.
Redness spreading beyond the spot
Can be irritation from the liquid running, and it should be looked at.

In Deeper Skin Tones

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Cantharidin works the same way on every skin tone. The difference is what is left behind while the skin heals.

Any blistering inflammation can trigger post-inflammatory hyperpigmentation — a flat brown or gray mark where the skin was inflamed. It is common after cantharidin on deeper skin tones, it is not a scar, and it usually fades over weeks to a few months. A pale mark can also appear, and that takes longer to even out.

Practical things that reduce it: treating fewer lesions at the first visit, a shorter contact time where the provider judges that appropriate, keeping the blister intact and covered, and protecting healed spots from sun until the color evens out. Some providers add a pigment-lowering cream once the skin has healed.

Keloid risk is low: The split is shallow and does not reach the deeper layer where keloids form. Still say at the first visit if you or your child has ever formed one.

Knowing the mark is coming: Families told in advance that a temporary mark is expected wait it out. Families who are not often stop treatment believing it has scarred.

If You Stop

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Cantharidin is not a maintenance treatment. Cleared lesions stay cleared.

Stopping partway: This means going back to waiting. Molluscum clears on its own eventually — often within about a year, sometimes considerably longer — so untreated bumps will still go on their own schedule, and can spread meanwhile. Warts are less predictable; some resolve, some persist for years.

Clearing it is not immunity: A child can catch molluscum again from another child, shared towels, or a sibling who still has it. Treating one child while another in the house is untreated often means it comes back around.

Why families stop: Usually the blisters, the number of visits, or a child who has had enough. All reasonable, and watchful waiting is a legitimate plan rather than giving up.

Combining Treatments

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Same day
Eczema creams
Moisturizer and a short anti-inflammatory course go on nearby eczema, not the bumps.
Same day
Curettage
Stubborn single lesions are sometimes removed under numbing cream in older children.
Pause a few days
Salicylic acid
Used daily at home for warts, paused either side of a visit.
Pause a few days
Retinoids and scrubs
Stop these before a visit, since thinned skin blisters more.
Wait 2 weeks
Treating the same spot
Let the blister heal fully, usually 2 weeks.
Wait 2–4 weeks
Another cantharidin visit
At least 2 weeks apart, more commonly 3 to 4.
Don’t combine
Cryotherapy
Providers may alternate the two, never on the same lesion the same day.

Insurance Coverage

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The FDA-approved product is considerably more expensive, and what you pay depends on your insurance and how it is billed. Some plans cover it, some do not.

Ask Your Doctor

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If you are pregnant or breastfeeding
Ask the doctor managing your pregnancy or breastfeeding.
If your child is under 2
Ask which product is used and why, since the FDA-approved one is approved from age 2.
If your child has eczema near the bumps
Ask whether the eczema should be settled first. Broken skin absorbs more and blisters harder.
If you have diabetes or poor circulation
Blistered skin heals slowly and gets infected more easily, so ask whether this is right.
If the lesions are on the face
Or the eyelids or genital skin. Cantharidin is not applied there — ask what else can be used.
If a previous blister was very large
Or very painful. Say so, so contact time or the number of lesions can be reduced.
If you cannot wash it off on time
Say so before it is applied. The timing is the safety step, and another treatment may suit better.
If you have ever formed a keloid scar
Mention it even though the risk here is low.
If you have a deeper skin tone
Ask what marks to expect and how long they take to fade.
If someone else in the house has molluscum
Ask how to handle towels, baths and bedding, because reinfection between siblings is common.
If you have any allergy
Mention it, along with any past cantharidin treatment that blistered unusually badly.

At-Home Versions

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Covering the bumps and not scratching
Good alternative
Molluscum spreads by scratching and by direct contact. Keeping bumps covered with clothing or a plaster, keeping nails short, and treating any eczema limits the spread while the immune system catches up.
Salicylic acid wart treatments
Good alternative
For warts. Liquids, gels and medicated plasters used daily for weeks to months. Painless, cheap, and the closest thing to a home version for warts. It is not a molluscum treatment.
Potassium hydroxide solution
Okay alternative
A 5 to 10 percent solution, for molluscum, sold over the counter in some countries. Dabbed onto each bump until it reddens. It stings, and overusing it irritates the skin or leaves a mark, so follow the directions closely.
Not sharing towels, flannels and baths
Okay alternative
Simple household measures that reduce passing molluscum between siblings. Worth doing even during treatment.
Duct tape for warts
Okay alternative
Trials have been mixed. Harmless and cheap, so reasonable to try alongside salicylic acid, but do not expect much on its own.
Cantharidin bought online
Bad alternative
Do not. The contact time is the entire safety margin, applying it to the wrong site causes serious blistering, and swallowing it is poisonous.

How It Compares

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Watchful waiting
The same
That molluscum clears on its own. Costs nothing, involves no blisters, and is a legitimate plan. The trade-off is months to a couple of years of visible bumps that can spread in the meantime.
CryotherapyDiagram: how Cryotherapy (Liquid Nitrogen) works in the skin
Cryotherapy
The same
Fast and done entirely in the office with no wash-off to manage. It is genuinely painful during and immediately after, which limits it in young children, and it leaves a pale mark more often on deeper skin tones.
Curettage
The same
Scrapes the bumps off, often clearing many in one visit. Needs numbing cream and a child who can stay still, can be frightening, and carries a small risk of a scar at each site.
Potassium hydroxide solution
The same
Applied at home, so no office visits, and cheaper. It stings, and overuse irritates the skin or leaves a mark.
Paula's Choice 2% BHA exfoliant, La Roche-Posay Effaclar cleanser and a CeraVe psoriasis cream pumpLabeled cutaway diagram of a single skin pore, headed Salicylic Acid. The epidermis is a brick-textured band across the top and the dermis is pink below it, with a yellow oil gland at the base of the pore and a small cluster of bacteria inside it. A single dark navy callout line points into the top of the pore: decreases dead cells in pore. There are no other callouts, so the drawing shows one effect where the other ingredient diagrams show two or three.
Salicylic acid
Different job
For warts, weak for molluscum. Painless and cheap, done at home, but slow — weeks to months of daily use.
Imiquimod
Worse
Evidence does not support it for molluscum in children. Trials did not show a benefit, and it commonly irritates the skin, so it has largely fallen out of use for this.

Finding a Provider

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Cantharidin is applied by dermatologists, pediatric dermatologists, pediatricians, and physician assistants and nurse practitioners working with them. It is not sold to the public, and an esthetician or spa should not be offering it.

Ask which product is used, compounded or FDA-approved, and the contact time for that product, because they differ. Ask whether anything is being taped over the area, since occlusion makes the effect stronger. Ask what to do if a blister is large or painful, how to reach the office out of hours, and what the visit will cost.

Red flags: being sent home with the liquid to apply yourself, application to the face, eyelids or genital skin, no wash-off time in writing, taping lesions without telling you, treating a large number of lesions on a young child at a first visit, and no warning that a blister is coming.

Myths

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  • "It comes from Spanish fly, so it must be dangerous." Poisonous if swallowed, which is exactly why a provider applies it and you never take it home. On the skin, in the amount and time used, the effect is a local blister.
  • "It must hurt, so it is not really suitable for children." Applying it is painless, which is why it is used in children. The soreness comes later with the blister and is usually mild.
  • "Leaving it on longer will work better." It will not. Longer contact makes a bigger, more painful blister, without clearing more lesions.
  • "The blister means something has gone wrong." The blister is the treatment. Without it, nothing has happened.
  • "It will scar." Usually not The skin splits above the layer that rebuilds itself, so most sites heal without a scar. What is left is pigment, and picking is what changes that.
  • "Molluscum has to be treated." It does not It clears on its own. Treatment is usually for spreading, itching, discomfort, or awkward locations.

Questions Patients Ask

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Does it hurt?

Not when it is applied — nothing sharp touches the skin and no numbing is needed, which is why it suits young children. Hours later a blister forms and can be sore or itchy for a day or two.

How long do I leave it on?

Exactly as long as you are told. Contact times differ between the compounded solution and the FDA-approved product, and between molluscum and warts. Longer just makes a bigger, more painful blister.

Will it leave a scar?

Usually not. The skin splits above the rebuilding layer, so most spots heal without a scar. What lasts weeks to months is a flat brown, pink or pale mark — pigment, not scar tissue. Picking early is what turns a mark into a scar.

Why does my child keep getting new bumps?

Because the virus is still there. Cantharidin removes the bumps it is painted on, but molluscum keeps making new ones until the immune system recognizes it, which takes months. Hence repeating every few weeks.

Can it be used on the face?

No. Cantharidin is not applied to the face, eyelids, or mucous membranes such as the lips or genital skin, because a blister there is painful and risky. Ask what suits those locations.

Do we have to treat molluscum at all?

No. It clears on its own. Treatment is usually chosen when bumps are spreading quickly, itching, sore, in an awkward place, or affecting a child's activities.

References

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