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Policy statement
When Crocker, Zad, Milling and Lawson, 2010 commenced their research, they expected to show a link between the non-use of cycling helmets and a heightened risk of head injury. An early press release (Seaton, 2008) (removed by the authors, after the commentary on their paper was submitted for peer-review) claimed: “preliminary results of a year-long study that indicates cyclists are nearly twice as likely to suffer a brain injury if they are not wearing a bicycle helmet”.
However, as the research progressed, it became clear that the strongest link with head injury was not helmets but alcohol use by cyclists. Of 40 alcohol-affected cyclists, 57.5% had head injuries, compared to 29.5% of cyclists who had not used alcohol, a highly significant difference. In fact, the research found no significant relationship between helmet use and head injury for sober cyclists. It just so happened that all the intoxicated cyclists except one did not wear a helmet.
Kim, Kim, Ulfarsson and Porrello, 2007 also discovered a correlation between alcohol and helmet use. In their study, only 1 of 174 intoxicated bicyclists wore a helmet. Li and Baker, 1994, too, noted that intoxicated bicyclists may be less likely to wear helmets. Li, Baker, Smialek and Soderstrom, 2001 found that sober cyclists are 7 times more likely to wear a helmet than drunk cyclists.
Kim, Kim, Ulfarsson and Porrello, 2007 found that intoxication greatly increased the probability of a bicyclist suffering a fatal injury in a collision with a motor vehicle (by 174%). A Canadian coroner's review has shown that of 129 cyclists' deaths, toxicology tests were performed on 90 and of these one-third (30) showed evidence of the cyclists being under the influence of alcohol or drugs (Ontario, 2012). Olkkonen and Honkanen, 1990 found that an inebriated cyclist had 10 times the risk of injury than a sober cyclist.
Frank, Frankel, Mullins and Taylor, 1995, a study in Portland, Oregon, found that alcohol-intoxicated riders are considerably more likely than sober cyclists to be severely injured or killed. While only 15% of killed and hospitalized adult cyclists had elevated blood alcohol levels, half of the adult cyclists whose injuries were fatal had been intoxicated.
More specifically, Olkkonen and Honkanen, 1990 and Andersson and Bunketorp, 2002 found that intoxicated cyclists have a greater risk of head injury. According to Kraus, Fife and Conroy, 1987, over half the brain-injured bicyclists aged 15 and older who were blood alcohol tested were legally intoxicated.
Spaite et al, 1995 found that the prior consumption of alcohol by cyclists is highly associated with greater injury severity, longer hospitalization, and higher health care costs. In particular, intoxicated cyclists are much more likely to suffer major head injuries. It also happened that drunk cyclists were less likely to wear helmets. Spaite had previously shown that the collision circumstances for helmeted and unhelmeted cyclists are often different (Spaite et al, 1991).
Li, Baker, Smialek and Soderstrom, 2001 concluded that alcohol use while bicycle riding is associated with a substantially increased risk of fatal or serious injury.
Other studies reporting a greatly increased risk for intoxicated cyclists include Kwigizile, Sando and Chimba, 2012.
Research going back many years has shown that alcohol consumption is associated with most causes of head injury.
Galbraith, Murray, Patel and Knill-Jones, 1976 and Haddon, Valien, McCarroll and Umberger, 1961 found a strong correlation between alcohol and head injuries among pedestrians. According to the UK National Health Service, alcohol may be a factor in about 65% of adult head injuries (NHS, 2012).
No helmet research has considered alcohol use by cyclists as a possible confounder and no correction has therefore been made for it. Given the strong association between alcohol, crashes and head injury and the fact that intoxicated cyclists rarely wear helmets, the confounding effect of alcohol could render many of the predicted benefits of helmet use unreliable for cyclists who have not been drinking.
Andersson AL, Bunketorp O, 2002. Cycling and alcohol. Injury 2002;33(6):467-471. 
Crocker, Zad, Milling and Lawson, 2010
Crocker P, Zad O, Milling T, Lawson KA, 2010. Alcohol, bicycling and head and brain injury: a study of impaired cyclists' riding patterns. American Journal of Emergency Medicine 2010;28(1):68-72. 
Frank, Frankel, Mullins and Taylor, 1995
Frank E, Frankel P, Mullins RJ, Taylor N, 1995. Injuries resulting from bicycle collisions. Academic Emergency Medicine 1995;2(3):200-203.
Galbraith, Murray, Patel and Knill-Jones, 1976
Galbraith S, Murray WR, Patel AR, Knill-Jones R, 1976. The relationship between alcohol and head injury and its effect on the conscious level. Br J Surg 1976 Feb;63(2):128-30.
Haddon, Valien, McCarroll and Umberger, 1961
Haddon W, Valien P, McCarroll JR, Umberger CJ, 1961. A controlled investigation of the characteristics of adult pedestrians fatally injured by motor vehicles in Manhattan. Journal of Chronic Diseases 1961;14(6):655-678.
Kim, Kim, Ulfarsson and Porrello, 2007
Kim JK,Kim S,Ulfarsson GF,Porrello LA, 2007. Bicyclist injury severities in bicycle/motor vehicle accidents. Accident Analysis & Prevention 2007 Mar;39(2):238-51.
Kraus JF, Fife D, Conroy C, 1987. Incidence, severity and outcomes of brain injuries involving bicycles. American Journal of Public Health 1987;77:76-78..
Kwigizile, Sando and Chimba, 2012
Kwigizile V, Sando T, Chimba D, 2012. Modeling bicyclist injury severity: a focus on the impact of alcohol and drug use. Transportation Research Board 12-0057.
Li G, Baker SP, 1994. Alcohol in fatally injured bicyclists. Accident Analysis & Prevention 1994;26(4):543-548.
Li, Baker, Smialek and Soderstrom, 2001
Li G, Baker SP, Smialek JE,Soderstrom CA, 2001. Use of alcohol as a risk factor for bicycling injury. JAMA 2001;285(7):893-896.
Causes of a minor head injury. NHS Choices.
Olkkonen S, Honkanen R, 1990. The role of alcohol in non-fatal bicycle injuries. Accident Analysis & Prevention 1990;22(1):89-96.
Cycling Death Review 2006 - 2010. Office of the Chief Coroner, Ontario, 2012.
Seaton Family Hospitals News, Press Release 2008. Previously accessible at: http://www.seton.net/about_seton/news/2008/05/29/while_umc_brackenridge_research_links_bicycle_helmets_and_injury_alcohol_use_determined_more_dangerous.
Spaite DW, Murphy M, Criss EA, Valenzuela TD, Meislin HW, 1991. A prospective analysis of injury severity among helmeted and non helmeted bicyclists involved in collisions with motor vehicles. Journal of Trauma 1991 Nov;31(11):1510-6..
Spaite DW, Weist EA, David MD, Valenzuela TD, Judkins D, Meislin HW, 1995. A prospective investigation of the impact of alcohol consumption on helmet use, injury severity, medical resource utilization and health care costs in bicycle-related trauma. Journal of Trauma 1995;38(2):287-290.