Botulinum Toxin Type A.
For the treatment of chronic anal fissure†, an initial total dose of 5–20 units of botulinum toxin type A generally has been used. Injections should be made both lateral and distal to the fissure; local anesthesia before injection generally does not appear to be necessary. Either the internal or external anal sphincter may be injected with the toxin; the optimal injection site has not been determined. Some clinicians state that that injection of the internal sphincter is preferable because of the role of internal sphincter spasm in chronic anal fissure and the fact that it is it is easily palapated and injected; however, other clinicians prefer injection of the external sphincter because injection in this area allows diffusion of the toxin to the internal sphincter, while localization of botulinum toxin type A in the internal anal sphincter is not always possible and may allow diffusion of the drug into the intrasphincteric space, which may result in fecal and/or flatulence incontinence. Some clinicians state that injection into the puborectalis muscle (caused by too deep an injection) should be avoided as it may be associated with a high risk of incontinence.
Following injection of botulinum toxin type A in patients with chronic anal fissure, a reduction in pain has been reported within 2 days and sphincter tone is reduced for 4 weeks or longer. Limited data suggest that healing rates may be higher with higher doses. Repeat injections using the same initial dose or a higher dose appear to be safe and effective in patients who fail to heal after the initial injection or who have relapses.
http://www.medscape.com/druginfo/monographcid=med&drugid=9827&drugname=Botox+IM&monotype=monograph&secid=4
http://books.google.co.uk/books?id=2qT6cc-EPtIC&pg=PA95&dq=2.5+units+botox+anal+fissure