BACKGROUND: Benign masseter muscle hypertrophy is an uncommon clinical 
phenomenon of uncertain aetiology which is characterised by a soft swelling near 
the angle of the mandible. The swelling may on occasion be associated with 
facial pain and can be prominent enough to be considered cosmetically 
disfiguring. Varying degrees of success have been reported for some of the 
treatment options for masseter hypertrophy, which range from simple 
pharmacotherapy to more invasive surgical reduction. Injection of botulinum 
toxin type A into the masseter muscle is generally considered a less invasive 
modality and has been advocated for cosmetic sculpting of the lower face. 
Botulinum toxin type A is a powerful neurotoxin which is produced by the 
anaerobic organism Clostridium botulinum and when injected into a muscle causes 
interference with the neurotransmitter mechanism producing selective paralysis 
and subsequent atrophy of the muscle.
OBJECTIVES: To assess the effects of botulinum toxin type A in the management of 
benign bilateral masseter hypertrophy.
SEARCH STRATEGY: We searched the following databases in August 2008: the 
Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 
2008, issue 3); MEDLINE (via PubMed) (1950 to August 2008); EMBASE (via 
embase.com) (1980 to August 2008); and LILACS via BIREME. We searched two 
bibliographic databases of regional journals which may be expected to contain 
relevant trials (IndMED and Iranmedex) using free text terms appropriate for 
this review.
SELECTION CRITERIA: Randomised controlled clinical trials (RCTs) and controlled 
clinical trials (CCTs) comparing intra-masseteric injections of botulinum toxin 
versus placebo administered for cosmetic facial sculpting in individuals of any 
age with bilateral benign masseter hypertrophy, which had been self-evaluated 
and confirmed by clinical and radiological examination. We excluded participants 
with unilateral or compensatory contralateral masseter hypertrophy resulting 
from head and neck radiotherapy.
DATA COLLECTION AND ANALYSIS: Two review authors conducted screening of studies 
in duplicate and independently, and although no eligible trials were identified, 
the two authors had planned to extract data independently and assess trial 
quality using standard Cochrane Collaboration methodologies.
MAIN RESULTS: We retrieved 167 references to studies, none of which matched the 
inclusion criteria for this review and all of which were excluded.
AUTHORS' CONCLUSIONS: We were unable to identify any randomised controlled 
trials on the efficacy of intra-masseteric injections of botulinum toxin for 
people with bilateral benign masseter hypertrophy. The absence of high level 
evidence for the effectiveness of this intervention emphasises the need for 
well-designed, adequately powered, randomised controlled clinical trials (RCTs) 
and controlled clinical trials (CCTs).