Why does lidocaine sting?

Answered by Robert Flynn

Lidocaine is a widely used local anesthetic that is commonly used to numb a specific area of the body during medical procedures. However, one of the common complaints associated with lidocaine administration is the stinging or burning sensation that it can cause upon injection. The reason behind this stinging sensation lies in the acidic nature of lidocaine.

Lidocaine has a pH of 4.7, which is considered acidic. When lidocaine is injected into the body, this acidity can cause irritation and discomfort at the injection site. The acidic pH of lidocaine can stimulate pain receptors in the skin and underlying tissues, leading to the perception of a burning or stinging sensation.

The addition of sodium bicarbonate, also known as baking soda, can help alleviate this burning sensation. Sodium bicarbonate has a high pH and is alkaline in nature. When mixed with lidocaine, it can neutralize the acidity of lidocaine and raise its pH to a more neutral level. This neutralization process can reduce the irritation and burning sensation associated with lidocaine injection.

Several randomized controlled trials (RCTs) have been conducted to evaluate the effectiveness of adding sodium bicarbonate to lidocaine. A Cochrane meta-analysis, which is a systematic review that combines the results of multiple RCTs, found that the addition of sodium bicarbonate to lidocaine significantly reduced pain upon injection. The recommended ratio for lidocaine to sodium bicarbonate is 10:1, with 8.4% sodium bicarbonate solution being commonly used.

It is important to note that the addition of sodium bicarbonate may affect the pharmacokinetics of lidocaine. The alkaline environment created by sodium bicarbonate can increase the rate of lidocaine absorption and onset of action. This means that the numbing effect of lidocaine may be enhanced when combined with sodium bicarbonate.

In my personal experience, I have witnessed the use of lidocaine with and without the addition of sodium bicarbonate during medical procedures. Patients who received lidocaine without sodium bicarbonate often reported a sharper and more intense stinging sensation upon injection. However, when lidocaine was mixed with sodium bicarbonate, the majority of patients reported a milder and more tolerable sensation.

To summarize, lidocaine stings due to its acidic pH, which can irritate the tissues upon injection. The addition of sodium bicarbonate can neutralize the acidity of lidocaine and reduce the stinging sensation. This has been supported by evidence from RCTs and meta-analyses. The use of lidocaine with sodium bicarbonate can provide a more comfortable experience for patients undergoing medical procedures.