[IOM] = Institute of Medicine, 1999 Marijuana and Medicine: Assessing the Science Base (Commissioned by the White House Office of National Drug Control Policy)
Potency is a red herring. Average use today may be of marijuana that is about 2 to 3 times as potent as decades ago but users simply use less to get the same effect with less smoke.
Even 30 years ago, much more potent strains than are normally used today were available, but were seldom used. As with alcohol and tobacco, users generally prefer milder versions.
A respected annual University of Michigan study (MTF) asks respondents about levels of intoxication:
'The line for marijuana is flat as a pancake. Kids who get stoned today aren't getting any more stoned than their parents were. That ought to be the end of the argument.' - UCLA professor Mark Kleiman (more below)
"Marijuana potency has increased; however, it is unlikely that average potency levels will reach 20 or 30 percent THC in the near term. Even with the advances in indoor cultivation techniques or marijuana production methods used throughout the United States and Canada where much of the higher potency marijuana is produced, THC levels remain, typically, under 15 percent. ... average THC levels likely will continue to increase only gradually or remain relatively stable." - NDIC Comment on NDTA 2005
Potency above 15 percent is quite unusual (although more potent strains have been around for decades). This contrasts with marijuana in pill form which is 100 percent potent yet has been reclassified from Schedule II to Schedule III meaning it became thought of as less dangerous. There is no known user market for the non-medical use of the pill which has been around for many years.
"A 2002 RCMP report cautions that exaggerated claims about marijuana potency are being made in the media and warns that any such claims should be based on "actual laboratory analysis results." The report provides the results of two such analyses. Between 1996 and 1999, a total of 3,160 samples of seized marijuana were tested for THC levels: The average each year varied between 5.5 to six per cent; the top-rated sample was 25 per cent, but samples of more than 16 per cent were extremely rare, and "almost a third of the samples were under three per cent.
"The THC content of all pot last year was 5.32 percent; during the past decade, it averaged 4.1 percent."
" [UCLA professor Mark] Kleiman cites the respected annual University of Michigan study that asks respondents about levels of intoxication. Writes Kleiman: 'The line for marijuana is flat as a pancake. Kids who get stoned today aren't getting any more stoned than their parents were. That ought to be the end of the argument.'
"Of course, the Walters scare campaign is nothing new. Back in 1994, City University of New York professor and marijuana advocate John Morgan cited three New York Times articles warning of alarming increases in marijuana's potency. They were published in 1980, 1986, and 1994."
- From "The Myth of Potent Pot"
The drug czar's latest reefer madness: He claims that marijuana is 30 times more powerful than it used to be.
By Daniel Forbes, November 19, 2002
Also see: [c]
"According to the federal Potency Monitoring Project, the average potency of marijuana has increased very little since the 1980s. The Project reports that in 1985, the average THC content of commercial-grade marijuana was 2.84%, and the average for high-grade sinsemilla in 1985 was 7.17%. In 1995, the potency of commercial-grade marijuana averaged 3.73%, while the potency of sinsemilla in 1995 averaged 7.51%. In 2001, commercial-grade marijuana averaged 4.72% THC, and the potency of sinsemilla in 2001 averaged 9.03%."
- Quarterly Report #76, Nov. 9, 2001-Feb. 8, 2002, University of Mississippi Potency Monitoring Project, NIDA Marijuana Project (NIDA Contract #N01DA-0-7707).
http://www.drugwarfacts.org/marijuan.htm [Item 29.]
The key point is that if much more potent cannabis is a problem - and it may be - the problem is largely a result of a prohibition that encourages more childhood experimentation, more potent strains, a lack of regulation that deprives users of needed knowledge and a lack of the cultural norms that encourage moderate use.
EXTRA-HIGH CANNABIS THEORY GOES UP IN SMOKE
The Guardian (UK), 6-26-04
The effective strength of cannabis consumed in Britain has remained stable for the past 30 years...
The research says there is no evidence for claims that most cannabis consumed in Britain and the rest of Europe is now 10 times or more stronger than it was in the 70s.
The US drugs "tsar" John Walters and toxicologist John Henry of St Mary's hospital in Paddington, west London, are among those who have warned that the cannabis available now bears little resemblance to that on the market 30 years ago, with serious health dangers for regular users.
The EU study says that the strength of the active ingredient - THC - has remained unchanged at about 6% for most of the cannabis smoked in Britain.
"There has been much speculation on the strength of cannabis available today, but little in the way of hard evidence," said its director, Georges Estievenart.
The report concludes it is possible that regular use of higher potency cannabis could lead to health problems such as panic attacks and minor psychological problems, but as yet this kind of cannabis remains relatively rare.
"Statements in the popular media that the potency of cannabis has increased by ten times or more in recent decades are not supported by the limited data that are available from either the USA or Europe."
"The THC content of cannabis products in general is extremely variable ... there have always been some samples that have had a high potency."
- European Union Monitoring Centre on Drugs and Drug Addiction, "EMCDDA Insights 6: An Overview of Cannabis Potency in Europe." 6-26-04
Almost certainly not at anything approximating normal use. Epidemiological studies show no difference in life expectancy for those who do and do not smoke marijuana.
"There is no conclusive evidence that marijuana causes cancer in humans, including cancers usually related to tobacco use." [IOM]
"In addition, tobacco smokers generally smoke considerably more cigarettes per day than do marijuana smokers." [IOM]
"The development of lung cancer requires a long exposure to smoking, and most marijuana users quit before this level of exposure is achieved." [IOM]
Cancer from tobacco normally seems to require 20 to 40 years of use of about a pack a day. A carton of cigarettes will last the average smoker a week or so but the same carton filled with marijuana would last the average marijuana smoker more than a year. Even though marijuana smoke may contain some five times the carcinogens found in tobacco smoke, this does not begin to approach the risks of tobacco due to the vast disparity in consumption. Also, most marijuana smokers choose to stop using it well before the age of 40.
If a current trend toward the use of vaporizers continues to grow, smoke inhalation and associated risks will be sharply reduced.
C. How addictive is marijuana ?
Not very when compared to other drugs.  Once a user decides to give up marijuana, which usually happens between ages 25 and 34, most do so with little difficulty.
For decades until recently, the government reports never even referred to marijuana as addictive. Now that the movement for marijuana reform has grown, the government has chosen to counter-attack with fear by distorting the definition of addiction (and the significance of potency) with reports that millions now suddenly need treatment. Many of these are coerced into "voluntary" requests for treatment.  Consider if a teen was found to have drunk a beer and was forced to admit to "alcohol dependence" and to enter a recovery program to avoid punishment.
You can love your car and you can love your mother, but the same word, "love" has very different meanings in context. The same applies to vague terms like "addiction," "drug dependence" and "chronic use." The Institute of Medicine report points out the relatively mild nature of marijuana "addiction." 
There is a growing body of evidence that marijuana can be used to help counter other drug addictions. 
European marijuana has normally been about 6% to 8% THC for 30 years, stronger than in the US but less than the recent use of 16% THC in The Netherlands. Strains may range - as they always have, both in Europe and the US - up to around 30% THC. Hashish oil may reach 70% THC.
To understand US propaganda about massive amounts of addiction to marijuana, it is useful to compare with The Netherlands where use has been de facto legal for over 20 years. They have some 3,500 treated for marijuana dependence yearly. This might roughly equate to 50,000 in the US but the US government reports millions since so many who are not in the least dependent are forced into treatment to avoid a variety of punishments -- 700,000 annual marijuana arrests in action in addition to school and parent generated treatment. This works out well for the propaganda mill and less scrupulous treatment providers. It does more harm than good for users who are not actually dependent and costs parents and users a great deal of wasted money.
... Roel Kerssemakers, spokesman for the Jellinek Clinic in Amsterdam, the main authority on addiction in the Netherlands. He says there's no scientific evidence to support claims that higher THC levels increase the risk of addiction.
"No, that is not proven. Our research shows that people using cannabis with a high amount of THC smoke a little bit less.
They like the effect, but don't need anymore THC and therefore smoke less, which is one of the positive sides of high levels of the substance. Research on animals shows that when given strong cannabis, they also tend to take less. "Every year, we treat around 3,500 people with cannabis addiction, and this number has been quite stable."
- From "Dutch cannabis under review" by Radio Netherlands Wereldomroep, April 8, 2004
It is not recommended but the research indicates minimal risks, particularly when compared to alcohol.
Risks may be considerably greater for an inexperienced user or even for an experienced user who has not practiced the task while under the influence.
Includes graph of California traffic fatalities in years after passage of medical marijuana law in 1996.
Considerably less intoxicating than alcohol and many other drugs. Experience gives users a greater ability to control their reactions.
See: www.drugwarfacts.org/addictiv.htm and note Column I
Intoxication tends to promote caution [See: Marijuana and Driving] and to suppress violence and anti-social behavior
Nor does marijuana use seem to have any significant impact on long term mental function :
"In conclusion, our meta-analysis of studies that have attempted to address the question of longer term neurocognitive disturbance in moderate and heavy cannabis users has failed to demonstrate a substantial, systematic, and detrimental effect of cannabis use on neuropsychological performance. It was surprising to find such few and small effects given that most of the potential biases inherent in our analyses actually increased the likelihood of finding a cannabis effect."
- Grant, Igor, et al., "Non-Acute (Residual) Neurocognitive Effects Of Cannabis Use: A Meta-Analytic Study," Journal of the International Neuropsychological Society (Cambridge University Press: July 2003), 9, p. 687.
www.drugwarfacts.org/marijuan.htm [Item 7.]
A Johns Hopkins study published in May 1999, examined marijuana's effects on cognition on 1,318 participants over a 15 year period. Researchers reported "no significant differences in cognitive decline between heavy users, light users, and nonusers of cannabis." They also found "no male-female differences in cognitive decline in relation to cannabis use." "These results ... seem to provide strong evidence of the absence of a long-term residual effect of cannabis use on cognition," they concluded.
- Constantine G. Lyketsos, Elizabeth Garrett, Kung-Yee Liang, and James C. Anthony. (1999). "Cannabis Use and Cognitive Decline in Persons under 65 Years of Age," American Journal of Epidemiology, Vol. 149, No. 9.
www.drugwarfacts.org/marijuan.htm [Item 9.]
NO. But making marijuana illegal creates a gateway of sorts, the reverse of our intentions.
The development of a vapor route for THC delivery offers promise for the future of medical marijuana research. A recent study found that THC administered through the Volcano® vaporizer resulted in higher plasma THC levels than smoked marijuana at both 30 and 60 minutes after administration. It also found that exhaled carbon monoxide increased very little after vapor compared with smoking (37). Those findings, along with patient preference for the vapor method, indicate opportunities for future clinical trials. Vaporization of THC offers the rapid onset of symptom relief without the negative effects from smoking. It allows patients to self- regulate their dosage immediately by ceasing inhalation when or if psychoactive effects become unpleasant. Scientists are also developing a pulmonary dronabinol to be delivered with a pressurized metered-dosed inhaler. Preliminary studies show rapid absorption, but FDA approval remains distant.
"We acknowledge that there is no clear alternative for people suffering from chronic conditions that might be relieved by smoking marijuana such as pain or AIDS wasting," [IOM]
There are no meaningful risks associated with medical use for most people. The smoke is in many ways undesirable but is also currently the most beneficial form of use for many.
"The side effects of cannabinoid drugs are within the acceptable risks associated with approved medications. Indeed, some of the side effects, such as anxiety reduction and sedation, might be desirable for some patients." [IOM]
The only major remaining medical issues are to better determine the range and efficacy of use and to develop a variety of alternate forms to smoked delivery.
Only political perversity stands in the way of a medicine that may benefit millions of people. Politics also impedes developing a medicine that holds promise for treatment of many conditions beyond those currently recognized and which may impact the medical care of tens of millions more.
For a more detailed discussion, see: "Are Texans Being Denied Access to a Vital Medicine? A Scientific Assessment of Marijuana"