Understanding the Side Effects of Prilocaine

Prilocaine is a local anesthetic you’ve probably never heard of-until you need it. It’s the numbing agent used in dental fillings, minor skin procedures, and sometimes during childbirth. Unlike lidocaine, which most people recognize, prilocaine is quieter, less talked about, and often assumed to be just as safe. But that’s where things get tricky. While it works well for blocking pain, prilocaine carries a unique risk that few patients are warned about: methemoglobinemia. This isn’t a rare side effect. It’s a real, measurable danger that can turn your blood from bright red to chocolate brown, leaving you short of breath, dizzy, or worse.

What Prilocaine Actually Does

Prilocaine blocks nerve signals in your body, stopping pain signals from reaching your brain. It’s often mixed with epinephrine to make the numbing last longer and reduce bleeding during procedures. Dentists use it for root canals. Dermatologists use it before removing moles or stitching cuts. Anesthesiologists sometimes use it in epidurals. It’s fast-acting, low in toxicity compared to other anesthetics, and breaks down quickly in the body. That’s why it’s popular.

But here’s the catch: prilocaine breaks down into a chemical called o-toluidine. This compound interferes with hemoglobin-the protein in your red blood cells that carries oxygen. Normally, hemoglobin holds onto oxygen and releases it where your body needs it. When o-toluidine gets in the way, it changes hemoglobin into methemoglobin, which can’t carry oxygen at all. Your blood still looks red, but it’s not doing its job.

The Hidden Risk: Methemoglobinemia

Methemoglobinemia is the most serious side effect tied to prilocaine. It doesn’t happen to everyone. In fact, most people get a small amount of prilocaine and never notice anything wrong. But when it does happen, it can be dangerous. The risk goes up with higher doses-like when you get multiple injections in one visit, or if you’re given prilocaine for a large area like a full back procedure.

Symptoms usually show up within minutes to an hour after the injection. You might feel unusually tired. Your lips or fingertips could turn blue or gray. You might get a headache, feel dizzy, or notice your breathing is shallow. In severe cases, your skin turns a muddy brown color. Your pulse oximeter-the machine that checks your oxygen levels-might read normal, but you’re still not getting enough oxygen. That’s because those machines can’t tell the difference between normal hemoglobin and methemoglobin. Only a blood test can confirm it.

Studies show that methemoglobinemia from prilocaine happens in about 1 in 1,000 cases when doses exceed 400 mg. That’s more than most people get in a single dental visit. But if you’re getting a large skin procedure, like removing several moles or a tattoo, you could easily hit that threshold. Children and older adults are more vulnerable. People with certain genetic conditions, like G6PD deficiency, are at higher risk too.

Who Should Avoid Prilocaine

If you’ve ever had a bad reaction to a local anesthetic, or if you’ve been told your blood doesn’t carry oxygen well, you should speak up before any procedure. You don’t need to know the term methemoglobinemia-you just need to say, "I’m worried about side effects from numbing shots. Has this been used safely in people like me?"

Prilocaine is not recommended for:

• Infants under 6 months
• People with known methemoglobinemia
• Those with G6PD deficiency
• Pregnant women in the third trimester, unless absolutely necessary
• People taking medications that affect oxygen delivery, like nitroglycerin or certain antibiotics

Even if none of these apply to you, it’s worth asking your provider if they’ve considered alternatives. Lidocaine, articaine, or bupivacaine are just as effective for most procedures and don’t carry the same methemoglobinemia risk.

What Happens If You Develop Methemoglobinemia

If you start feeling off after a prilocaine injection, don’t wait. Tell someone right away. If you’re at a clinic, alert the nurse or doctor. If you’re at home, call emergency services. This isn’t something you can treat yourself.

The treatment is simple if caught early: methylene blue. It’s a blue dye that’s been used since the 1930s to reverse methemoglobinemia. A single IV dose can restore your blood’s ability to carry oxygen within minutes. In mild cases, oxygen therapy alone may be enough. In severe cases, you might need a blood transfusion.

Most people recover fully if treated quickly. But if it’s missed, the lack of oxygen can damage your heart, brain, or other organs. There are documented cases where people went into cardiac arrest because methemoglobinemia wasn’t recognized. That’s why it’s so important to know the signs-even if you think you’re fine.

Alternatives to Prilocaine

You don’t have to accept prilocaine just because it’s the default option. Many clinics use it because it’s cheap and widely available. But there are better choices for safety.

Lidocaine is the most common alternative. It’s been used for over 70 years. It doesn’t break down into o-toluidine, so it doesn’t cause methemoglobinemia. Articaine is another option-it’s slightly stronger and faster-acting than lidocaine, and it’s widely used in Europe and Australia. Bupivacaine lasts longer, making it ideal for procedures where pain relief is needed for hours after.

Some providers still use prilocaine because they’ve always used it. But if you ask, most will switch without hesitation. You’re not being difficult-you’re being smart. Ask: "Is there another anesthetic that doesn’t carry the risk of methemoglobinemia?" If they say no, ask why. If they can’t explain, it’s time to find another provider.

What to Do Before Your Next Procedure

Before you get any local anesthetic, take five minutes to prepare:

1. Ask what drug they’re using. Don’t assume it’s lidocaine.
2. Ask if it’s prilocaine. If yes, ask why.
3. Ask if there’s a safer alternative.
4. Tell them if you’ve ever had trouble with oxygen or unusual reactions to medicine.
5. If you’re getting multiple injections or a large area numbed, ask about total dosage limits.

Don’t be shy. This is your body. You have the right to know what’s going in it. Most providers appreciate the question. They’ve seen patients panic because they didn’t know what was happening. You’re preventing that.

Real Cases That Should Make You Pause

In 2023, a 68-year-old man in Queensland had three dental procedures done in one day. He got prilocaine each time. By the afternoon, he was dizzy and his lips turned blue. He went to the ER. His methemoglobin level was 28%-normal is under 1%. He needed methylene blue and stayed overnight. He’d never heard of prilocaine before.

In 2022, a newborn in Melbourne developed methemoglobinemia after a circumcision with prilocaine. The baby turned gray and stopped breathing. The medical team acted fast, but the child spent three days in intensive care. The hospital changed its protocol after that.

These aren’t outliers. They’re predictable. And they’re preventable.

Bottom Line: Prilocaine Isn’t Dangerous-But It’s Not Risk-Free

Prilocaine works. It’s not a bad drug. But it’s not the safest choice for everyone. The risk of methemoglobinemia is low, but when it happens, it’s serious. And unlike most side effects, it’s invisible until it’s too late.

If you’re getting a small procedure-like a filling or a mole removal-and you’re healthy, the odds are in your favor. But if you’re getting multiple shots, a large area numbed, or you have any underlying health issues, you should insist on an alternative. Lidocaine is just as effective. Articaine is just as common. There’s no reason to gamble with your oxygen supply.

Knowledge is your best defense. Ask the question. Know the signs. Don’t let silence be your only safety net.