How Numbing Creams Work
Topical anesthetics, commonly known as numbing creams, function by temporarily blocking nerve signals in the skin [1.2.1]. They contain active ingredients like lidocaine, benzocaine, tetracaine, or prilocaine [1.7.1, 1.7.2]. These molecules penetrate the skin's outer layer, the stratum corneum, to reach nerve fibers in the epidermis and dermis. Once there, they inhibit voltage-gated sodium channels, which stops the nerves from sending pain signals to the brain [1.2.1]. This mechanism provides localized pain relief for procedures such as injections, laser treatments, tattooing, and minor surgeries [1.2.1, 1.3.3].
Systemic Absorption: Does It Enter the Bloodstream?
Yes, the active ingredients in numbing creams can and do get absorbed through the skin and enter the bloodstream [1.3.2, 1.3.5]. This process is known as systemic absorption. While these products are designed for local effects, the body's circulatory system can pick up the drug from the application site. The skin acts as a protective barrier, but it is not impermeable [1.4.2]. The rate and extent of absorption depend on numerous factors, and when absorption is excessive, it can lead to toxic levels of the anesthetic in the blood, causing potentially life-threatening side effects [1.3.1, 1.3.5]. The U.S. Food and Drug Administration (FDA) has issued warnings about this risk, especially when products are used without medical supervision for cosmetic procedures [1.3.1].
Factors Influencing Bloodstream Absorption
Several variables can significantly increase the amount of anesthetic that enters your bloodstream [1.4.1, 1.4.2]:
- Skin Integrity: This is the most critical factor. Applying numbing cream to broken, irritated, inflamed, or damaged skin dramatically increases absorption [1.4.3, 1.11.4]. Conditions like eczema, psoriasis, rashes, or open wounds compromise the skin's protective stratum corneum, allowing the drug to bypass this barrier and enter the system more readily [1.2.1, 1.4.2].
- Surface Area: The larger the area of skin the cream is applied to, the greater the potential for systemic absorption [1.3.4, 1.4.3]. Applying cream to an entire leg, for instance, poses a much higher risk than treating a small patch on the face [1.3.1].
- Duration of Application: Leaving the cream on for extended periods allows more time for the drug to penetrate the skin and enter circulation [1.3.3, 1.4.3]. It is crucial to follow the recommended application time.
- Occlusion: Covering the treated area with a dressing or plastic wrap (occlusion) traps heat and moisture, which increases skin hydration and permeability [1.9.1]. This technique enhances the numbing effect but also significantly boosts systemic absorption and the risk of toxicity [1.3.1, 1.9.1].
- Concentration of Anesthetic: Products with higher concentrations of active ingredients, such as compounded creams, deliver more drug to the skin, increasing the amount available for absorption [1.4.4, 1.8.4].
- Heat: Applying heat to the area, whether from a heating pad, sunbathing, or exercise, increases skin temperature and blood flow, which can accelerate absorption into the bloodstream [1.3.1, 1.10.1].
Risks and Symptoms of Systemic Toxicity
When high levels of anesthetic enter the bloodstream, it can lead to Local Anesthetic Systemic Toxicity (LAST). This is a serious condition that affects the central nervous system (CNS) and cardiovascular system [1.2.1, 1.4.5].
Early or mild symptoms of CNS toxicity may include:
- Numbness or tingling around the mouth (circumoral numbness) [1.2.1]
- Metallic taste in the mouth [1.5.4]
- Ringing in the ears (tinnitus) [1.5.1]
- Dizziness or lightheadedness [1.5.4]
- Blurred or double vision [1.5.1]
- Confusion, anxiety, or agitation [1.5.1, 1.5.4]
As toxicity progresses, more severe symptoms can occur:
- Muscle twitches and tremors [1.5.1]
- Seizures [1.3.5]
- Drowsiness leading to loss of consciousness or coma [1.3.3, 1.5.1]
- Slow or shallow breathing, or respiratory arrest [1.5.1, 1.5.2]
Cardiovascular symptoms include:
- Irregular heartbeat (arrhythmias) or slow heart rate (bradycardia) [1.3.5, 1.5.4]
- Low blood pressure (hypotension) [1.5.4]
- In severe cases, cardiac arrest [1.3.3, 1.4.5]
Another rare but serious risk associated with some topical anesthetics (especially benzocaine) is methemoglobinemia, a blood disorder where hemoglobin cannot effectively carry oxygen. This can cause symptoms like pale or blue-colored skin, shortness of breath, and fatigue [1.2.1, 1.10.2].
Comparison of Common Topical Anesthetics
Different anesthetics have different properties, affecting their onset, duration, and potential for side effects. Amide-type anesthetics (like lidocaine and prilocaine) are metabolized in the liver, while ester-type anesthetics (like benzocaine and tetracaine) are broken down in the plasma [1.2.1]. This difference can be important, especially for individuals with liver conditions.
| Anesthetic | Type | Onset of Action | Duration of Action | Key Considerations |
|---|---|---|---|---|
| Lidocaine | Amide | Rapid (3-5 minutes) [1.2.1] | Intermediate (approx. 4 hours) [1.2.1] | Widely used; metabolized by the liver [1.2.1]. |
| Benzocaine | Ester | Fast-acting [1.8.1] | Up to 3 hours [1.3.3] | Often found in OTC products; higher risk of allergic reactions and methemoglobinemia [1.2.1]. |
| Tetracaine | Ester | Slower | Long (4-6 hours) [1.2.1] | Potent and offers prolonged numbing; often combined with other agents [1.8.1]. |
| Prilocaine | Amide | Very Rapid (2-4 minutes) [1.2.1] | Shorter (1-1.5 hours) [1.2.1] | Carries a risk of methemoglobinemia, especially in infants [1.2.1]. |
How to Use Numbing Creams Safely
To minimize risks and prevent dangerous levels of absorption, follow these safety guidelines:
- Use the Smallest Amount Possible: Apply only a thin layer of cream required to cover the treatment area [1.3.3, 1.6.2].
- Apply to Intact Skin Only: Never apply numbing cream to skin that is broken, scraped, blistered, or has a rash [1.3.3, 1.6.1].
- Limit the Application Area: Avoid using the cream on large sections of your body [1.9.1]. If a large area needs treatment, consider doing it in separate, smaller sessions [1.9.1].
- Adhere to Time Limits: Follow the product's instructions for how long to leave it on the skin. Do not exceed the recommended duration [1.3.3].
- Avoid Heat and Occlusion: Do not cover the area with plastic wrap or apply heat unless specifically instructed by a healthcare professional [1.3.1, 1.6.1].
- Consult a Professional: Discuss the use of numbing cream with your doctor before a procedure, especially if using a high-concentration or compounded product [1.3.2]. Be aware of your own risk factors, such as low body weight or underlying heart, liver, or CNS conditions [1.4.4].
Conclusion
While numbing creams are effective tools for pain management, their active ingredients can indeed enter the bloodstream. The risk of significant systemic absorption and subsequent toxicity is low when products are used correctly on small areas of intact skin for a limited time. However, this risk increases substantially when the cream is applied to large surface areas, broken skin, or for prolonged periods, especially with occlusion. By understanding the factors that drive absorption and adhering to safety guidelines, users can effectively minimize pain while avoiding potentially severe adverse effects.
For more information on the systemic risks of topical anesthetics, particularly for patients with compromised skin barriers, you can review this article from the National Institutes of Health: Systemic Risks of Topical Anesthetics in Barrier-Compromised Dermatologic Patients
