Twenty-three states and the District of Columbia have legalized medical marijuana. Colorado and Washington have legalized recreational use. As a result, the belief that marijuana use is "no big deal" has become increasingly pervasive in the United States.
Just over half of Americans support federal legalization of marijuana and, perhaps not surprisingly, half of these admit to trying it. Proponents of lifting the ban tout marijuana's medicinal properties and claim it is less harmful than tobacco or alcohol use and argue it should therefore join those two products as a taxable commodity.
Yet, if marijuana is as medicinal and harmless as advocates assert, why do over a dozen national health organizations, including the American Medical Association, the American Glaucoma Society and the National Multiple Sclerosis Society, oppose legalization even for medicinal purposes? (1)
In a word: science.
A recent article in the Journal of the American Medical Association noted there is very little scientific evidence to support the use of medical marijuana, never mind the alleged safety of recreational use. Authors Samuel Wilkinson and Deepak D'Souza explain that medical marijuana is considerably different from all other prescription medications in that "[e]vidence supporting its efficacy varies substantially and in general falls short of the standards required for approval of other drugs by the US Food and Drug Administration (FDA)."
Although some studies suggest marijuana may palliate chemotherapy-induced vomiting, cachexia in HIV/AIDS patients, spasticity associated with multiple sclerosis, and neuropathic pain, there is no significant evidence marijuana is superior to FDA approved medications currently available to treat these conditions. Additionally, support for use of marijuana in other conditions (such as post-traumatic stress disorder, Crohn's disease and Alzheimer's) is not scientific, relying on emotion-laden anecdotes instead of adequately powered, double-blind, placebo-controlled randomized clinical trials. 
Wilkinson and D'Souza go on to say that "[p]rescription drugs are produced according to exacting standards to ensure uniformity and purity of active constituents … Because regulatory standards of the production process vary by state, the composition, purity, and concentration of the active constituents of marijuana are also likely to vary. This is especially problematic because unlike most other prescription medications that are single active compounds, marijuana contains more than 100 cannabinoids, terpenoids, and flavonoids that produce individual, interactive, and entourage effects."  
As a consequence, there are no dosing guidelines for marijuana for any of the conditions it has been approved to treat. And finally, there is no evidence that the potential healthful effects of marijuana outweigh its documented adverse effects.
So that there can be no misunderstanding, physicians and health organizations who opposed medical marijuana do so on the basis of "First do no harm." If and when rigorous research delineates marijuana's true benefits relative to its hazards, compares efficacy with current medications on the market, determines optimal routes of delivery and dosing, standardizes its production and dispensing to match that of schedule II medications (like narcotics and opioids), then medical opposition will dissipate.
In other words, the ethical principles of non-malfeasance and beneficence demand that the federal ban remain in force to allow the necessary scientific investigation which states having legalized marijuana are well positioned to conduct.
And why do we need one? There are quite a few reasons. Marijuana is addictive. One in nine users overall, one in six adolescent users and 25-50 percent of daily users become addicted to marijuana. This means they develop tolerance to the drug needing more of it to achieve the same high, experience withdrawal symptoms in the absence of use, and continue to use it despite significant objective impairments to their health and lives. There is no such thing as "safe smoking." Like smoking tobacco, smoking marijuana causes cough, wheeze, increased phlegm production and emphysema. Although many allege marijuana is safer than cigarettes, a 2007 study determined that in terms of triggering airway obstruction (asthma), smoking one joint produced the same effect as smoking 2.5 - 5 cigarettes. There is also evidence that smoking two or more joints daily over several years results in decreased lung function. This is hardly surprising since marijuana contains many of the same chemicals and carcinogens found in tobacco. Some of these carcinogens, in fact, occur in greater concentrations in marijuana. Consistent with this, pre-cancerous changes in respiratory cells have been found in lung biopsies of chronic marijuana smokers, and a large Swedish study published in 2013 correlated heavy marijuana use to a greater than double increase in lung cancer risk over 40 years. Cancer risk: Further research is required to determine whether or not smoking and/or ingesting marijuana with all of its carcinogens causes cancer in other organ systems (just as smoking tobacco does). To date, studies have correlated smoking marijuana with an increased risk of head and neck cancers, bladder cancer and an aggressive form of testicular cancer. Cardiovascular effects: Marijuana increases heart rate by 20-100% shortly after smoking. Accelerated rates may last up to three hours. This translates into a greater risk of developing atrial fibrillation and other abnormal heart rhythms which may cause heart attacks and strokes. One study, for example, estimated marijuana users have a 4.8-fold increase in the risk of heart attack within the first hour of smoking the drug. There are also reports of deterioration of the heart muscle (cardiomyopathy) and increased angina among marijuana users. Mental effects:  Marijuana's immediate effects upon the brain are decreased concentration, impaired short-term memory, prolonged reaction time, drowsiness and lethargy. However, long-term irreversible adverse effects on the brain occur when marijuana use begins during adolescence.  These include impaired long-term memory, decrease in IQ and altered thinking. A large prospective longitudinal study in New Zealand found that subjects who used marijuana heavily in their teens and continued through adulthood showed a significant drop in IQ between the ages of 13 (pre-marijuana use) and 38 years of age. Among those who met criteria for marijuana dependence the average decrease in IQ was 8 points. For context, a loss of 8 IQ points could drop a person of average intelligence into the lowest third of the intelligence range.
In its policy statement opposing legalization, the American Academy of Child and Adolescent Psychiatry (AACAP) states that legalization of marijuana, even if restricted to adults, is likely to be associated  with increased adolescent access to and use of marijuana.  Indeed, there is much evidence this is happening, including the observation that calls to poison control centers for marijuana intoxication of youth increased 11.5 percent annually among states that enacted legislation between 2005 and 2011.
The AACAP notes further that "heavy use [during adolescence] is associated with [an] increased incidence and worsened course of psychotic, mood, anxiety, and substance use disorders across the lifespan.  Furthermore, marijuana's deleterious effects on adolescent brain development, cognition, and social functioning may have immediate and long-term implications, including increased risk of motor vehicle accidents, sexual victimization, academic failure, lasting decline in intelligence measures, psychopathology, addiction, and psychosocial and occupational impairment." Marijuana and pregnancy: Marijuana's anti-emetic effect is well known. Consequently, some pregnant women choose to self-medicate with marijuana to relieve morning sickness. This is tragic since children exposed to marijuana in utero sustain serious neurologic deficits: decreased memory and reasoning abilities, impaired visual task performance, attention deficits, and higher rates of hyperactivity, impulsivity, drug addiction and delinquency. These children also have an increased risk of developing brain cancer and leukemia which may be due to one or more of marijuana's many carcinogens. The multiplier effect: Finally, one must consider that there will be many marijuana users who will combine their marijuana use with alcohol and/or tobacco, thereby multiplying the likelihood of the occurrence of overlapping adverse health effects. Also, like nicotine and alcohol, marijuana primes the brain for a heightened response to other drugs. Second-hand smoke: There is no research regarding exposure to second-hand smoke from marijuana of which I am aware, but given the overlap of constituents between tobacco and marijuana, adverse effects very likely will be found should the research be undertaken.
To recapitulate, with increasing legalization, the view marijuana is a "safe way to get high" is prevalent and growing, especially among adolescents. While there is a need for further study of marijuana's over 100 active compounds, as well as its harms and potential benefits, the adverse effects presently documented are deeply concerning.
Increased availability has led to increased use by adults and children even though sales are technically restricted to adults. It is perfectly reasonable to hypothesize that if the federal ban is lifted, increased marijuana use will result in a greater number of individuals suffering from its untoward effects, and children will be disproportionately affected. This will deal a devastating blow to our long-term state and national economies by adversely affecting work-place productivity, academic performance, behavioral and medical health, automobile safety, parenting, and family functioning.
Science, not emotion-laden anecdotes or marijuana lobbyists, should determine the outcome of this debate. Parents, legislators, healthcare providers and other opinion-makers who value what is best for children and societal health will demand funding of scientifically rigorous long-term research untainted by those who benefit from the budding marijuana industry. Michelle Cretella MD is the Vice-President of the American College of Pediatricians and chairs the College’s Committee on Adolescent Sexuality. She is also a member of the American Academy of Pediatrics. Notes (1) The American Medical Association, American Society of Addiction Medicine, National Institute on Drug Abuse of the NIH, American Academy of Pediatrics, American College of Pediatricians, American Academy of Child and Adolescent Psychiatry, American Academy of Ophthalmology, American Glaucoma Society, National Multiple Sclerosis Society, National Comprehensive Cancer Network, American Cancer Society, and the Narcotics Enforcement Officers Association.