The Science of Transdermal Absorption
The transdermal delivery of medication through a patch is a controlled process designed to deliver a low, steady dose of medication directly to the application site. The skin, while an effective barrier, is not impenetrable. A small portion of the lidocaine from the patch is absorbed through the layers of the skin, ultimately reaching the tiny blood vessels in the dermis, which then carries it into the systemic circulation.
Studies have confirmed that when lidocaine patches (such as the prescription 5% patch) are used according to recommendations—for example, up to three patches worn for 12 hours—only a small fraction of the total lidocaine dose is absorbed systemically. In fact, research indicates that as little as 3% to 11% of the lidocaine is absorbed after a 12-hour application, with the majority remaining in the patch. This low systemic absorption is a key safety feature, as it allows for localized pain relief without the significant systemic exposure associated with oral medications.
Absorption Levels: Standard vs. Overuse
When patients follow the prescribed or over-the-counter (OTC) instructions, the systemic absorption of lidocaine is typically not a concern. Peak blood concentrations of lidocaine are well below the levels required to treat heart rhythm problems or cause systemic toxicity. For example, mean peak blood concentrations are reported to be around 0.13 mcg/mL, whereas toxic levels are typically considered above 5 mcg/mL.
However, this low-risk profile changes dramatically with misuse. Applying more patches than recommended, wearing them for extended periods beyond the 8-12 hour limit, or using them over a large surface area can lead to significantly increased systemic absorption. This can cause blood lidocaine concentrations to rise to dangerous levels, potentially triggering systemic adverse effects or even an overdose. The 12-hours-on, 12-hours-off regimen for many patches is specifically designed to prevent this accumulation of the drug in the bloodstream.
Factors That Increase Systemic Absorption
Certain conditions and practices can compromise the skin barrier and increase the rate and amount of lidocaine absorbed into the bloodstream. Patients should be aware of these factors to ensure safe use:
- Damaged or Broken Skin: Applying a patch to a cut, burn, rash, or any area of broken skin allows the lidocaine to be absorbed more quickly and directly into the bloodstream, bypassing the normal skin barrier. Always apply patches to clean, intact skin.
- External Heat Sources: Using heating pads, electric blankets, or even sunbathing over the patch can accelerate absorption. The heat dilates blood vessels in the skin, allowing more lidocaine to enter the circulation faster, raising the risk of high blood concentrations.
- Wrapping or Occlusion: Covering the patch with a tight bandage or non-porous material can trap heat and moisture, which can also increase absorption. Avoid tight bandaging unless instructed by a healthcare provider.
- Applying Multiple Patches: The dose absorbed is directly related to the surface area covered. Using more patches than directed can lead to a dangerously high total dose and increased systemic exposure.
Systemic Side Effects and Overdose Risks
When systemic lidocaine levels become too high, a patient may experience signs of overdose, a condition known as local anesthetic systemic toxicity (LAST). The symptoms range from mild to severe and can affect the central nervous system (CNS) and cardiovascular system.
Early signs of overdose include:
- Numbness or tingling around the mouth and tongue
- Dizziness or lightheadedness
- Tinnitus (ringing in the ears)
- Metallic taste in the mouth
- Nausea and vomiting
- Blurred vision
In more severe cases, symptoms can progress to:
- Confusion and disorientation
- Muscle tremors or twitching
- Seizures
- Decreased heart rate (bradycardia)
- Hypotension (low blood pressure)
- Respiratory depression or arrest
- Cardiac arrest
Comparison Table: Patches vs. Oral Medications
| Feature | Lidocaine Patches (Topical) | Oral Pain Medication (Systemic) |
|---|---|---|
| Absorption | Minimal systemic absorption; localized action | Entire dose absorbed into the bloodstream for widespread effect |
| Side Effects | Primarily local (skin irritation); systemic side effects are rare when used correctly | Potential for gastrointestinal, liver, and other systemic side effects |
| Area of Action | Targets a specific, localized area of pain | Provides general pain relief throughout the body |
| Onset | Works slowly over time, providing extended relief | Works relatively quickly for rapid pain relief |
| Drug Interactions | Lower risk of drug-drug interactions due to minimal systemic exposure | Higher risk of interactions as drugs are metabolized in the liver |
| Primary Use | Localized neuropathic pain (e.g., shingles) and minor muscle/joint pain | Acute pain (e.g., headache, injury) and widespread chronic pain |
Conclusion
Yes, lidocaine patches do get in your bloodstream, but this is a controlled process with very low systemic absorption under proper use. The transdermal delivery system is designed to provide localized pain relief, minimizing the medication's effect on the entire body. By adhering to dosing recommendations, avoiding application to broken skin or with external heat, and safely disposing of used patches, patients can effectively manage localized pain while mitigating the risks associated with higher systemic drug levels. For any concerns about medication or proper usage, always consult a healthcare professional. For more details on the safety and usage of lidocaine patches, resources like MedlinePlus offer comprehensive information.
