Adding sodium bicarbonate to local anaesthetic has all of the following effects, except:

The addition of sodium bicarbonate to local anesthetics improves the speed of onset, reduces the minimum concentration of the local anesthetic required and improves the quality of block. Adding sodium bicarbonate increases the pH of the local anesthetic solution, and thus more amount of free uncharged drug is available which can increase the rate of diffusion across the nerve sheath and improve the onset of anesthesia. Subcutaneous injections of local anesthetics are painful. Alkalinization of the solution by adding sodium bicarbonate before injecting can decrease the pain.