A sclerosing agent is a chemical substance injected into a blood vessel, cyst, or body cavity to deliberately damage the inner lining and cause the structure to collapse, scar shut, and eventually be reabsorbed by the body. These agents are the active ingredient behind sclerotherapy, one of the most common treatments for spider veins and varicose veins, though they’re also used to treat conditions ranging from hemorrhoids to fluid buildup around the lungs.
How Sclerosing Agents Work
When a sclerosing agent is injected into a vein, it damages the cells lining the vessel wall. This damage triggers the body’s clotting system, forming a small clot inside the vein. Over the following weeks, the body breaks down the clot, the vessel walls stick together, and scar tissue (fibrosis) replaces what was once an open channel. Blood flow naturally reroutes through healthier nearby veins. The treated vein gradually fades and is absorbed.
The same basic principle applies outside of veins. When injected into a cyst, the agent irritates the inner wall, causing it to scar and shrink. When used between the two layers of tissue surrounding the lungs (a procedure called pleurodesis), the chemical triggers inflammation that fuses the layers together, preventing fluid from accumulating in the space again.
Three Main Types of Sclerosing Agents
Sclerosing agents fall into three broad categories, each damaging tissue through a different mechanism.
- Detergents are the most widely used category for vein treatment. They work by disrupting the cell membranes lining the vessel wall. Polidocanol and sodium tetradecyl sulfate are the two most common, and both are FDA-classified sclerosing agents. Polidocanol also has a mild numbing effect, which makes injections less painful.
- Osmotic agents use highly concentrated salt or sugar solutions to draw water out of the cells lining a vessel, causing them to dehydrate and die. Concentrated saline (23.4%) is the most common example.
- Chemical irritants cause direct chemical injury to the vessel wall. These range from very mild formulations (glycerine-based mixtures) to aggressive ones (polyiodinated iodine, which can destroy a vessel wall in seconds). They’re chosen based on how much damage a particular situation calls for.
Liquid vs. Foam Formulations
Detergent sclerosing agents can be injected as either a liquid or a foam. The foam version is made by mixing the liquid agent with air or carbon dioxide, creating tiny bubbles that expand inside the vein and push blood out of the way. This gives the agent much better contact with the vessel wall.
The difference in effectiveness is dramatic for larger veins. In studies of the saphenous veins (the large veins running the length of the leg), foam sclerotherapy achieved success rates of 76% compared to just 40% for liquid. For smaller spider veins and reticular veins, both forms work well, but foam still holds an edge for anything beyond the smallest vessels.
Common Medical Uses
Vein treatment is the most familiar application. Spider veins (under 1 mm) and reticular veins (1 to 3 mm) are the primary targets, typically requiring small volumes of 0.1 to 0.3 mL per injection site, with multiple injections in a single session. The total volume per session generally stays at or below 10 mL. In a head-to-head trial, both polidocanol and sodium tetradecyl sulfate achieved treatment success rates above 90% at 26 weeks. Patient satisfaction ran higher with polidocanol: 84% reported being satisfied or very satisfied, compared to 63% for sodium tetradecyl sulfate.
Beyond leg veins, sclerosing agents treat a surprisingly wide range of conditions. They’re injected into hemorrhoids to shrink swollen tissue, into esophageal varices (dangerously enlarged veins in the throat) to stop or prevent bleeding, and into hydroceles and cysts (including Baker’s cysts behind the knee) to collapse fluid-filled sacs. For recurrent fluid buildup around the lungs, talc is the most commonly used sclerosing agent because it’s inexpensive and effective. Doxycycline, an antibiotic that also causes significant tissue irritation, is used as an alternative when talc isn’t suitable.
What to Expect After Treatment
For vein sclerotherapy, compression stockings are a standard part of recovery. There’s no single agreed-upon protocol: some practitioners recommend wearing them continuously (day and night) for the first few days to a week, then switching to daytime-only wear for several more weeks. Others prescribe a simpler regimen of daytime compression for two to six weeks. Pressure levels typically range from 18 to 40 mmHg depending on the size of veins treated. Your provider will give you specific instructions based on the extent of your procedure.
Most people return to normal activities within a day or two. Walking is usually encouraged right away, while strenuous exercise, hot baths, and prolonged sun exposure on the treated area are generally avoided for a couple of weeks.
Side Effects and Risks
The most common side effect is hyperpigmentation, a brownish discoloration of the skin along the treated vein. This occurs in roughly 7% to 33% of people treated with polidocanol, depending on the type of vein and formulation used. Most of this fading resolves on its own within several months, though foam sclerotherapy carries a higher chance of pigmentation lasting beyond a year (up to 17.5% of cases in some studies).
Temporary side effects like bruising, mild swelling, and small, firm lumps along the treated vein are common and typically resolve within weeks. Some people experience itching or a slight burning sensation at injection sites.
Rare but more serious complications include Nicolau syndrome, a condition where the injection causes localized tissue death, leading to pain, skin discoloration, and possible scarring. Deep vein clots, allergic reactions, and visual disturbances (particularly with foam formulations) are also possible but uncommon.
Who Should Not Receive Sclerotherapy
Sclerosing agents are not appropriate for everyone. Sclerotherapy is strictly off-limits for people with a known allergy to the specific agent, an active infection (either bodywide or at the treatment site), a history of deep vein thrombosis, severe peripheral artery disease, or advanced connective tissue diseases. Anyone who has experienced a heart attack, stroke, or neurological event following a previous sclerosant injection should not undergo the procedure again.
For hemorrhoid sclerotherapy specifically, additional contraindications include thrombosed hemorrhoids, a history of asthma, blood clotting disorders, inflammatory bowel disease, and significant heart, kidney, or liver problems.