Potency

If you're smoking a joint to enjoy the psychoactive effects of THC, then you might want to know how much of the potent cannabinoid is in a typical smoke. Here you'll learn how to calculate the amount of THC in a joint and understand what percentage of the cannabinoid your body is actually processing. 
The weight of the joint (in milligrams) multiplied by potency/THC percentage equals total milligrams of THC. 
One gram = 1000 milligrams. 
Example: The difference between a half gram and full gram joint of the same 20% THC flower:
1 gram: 1000 mg x 0.20 = 200 mg
Half gram: 500 mg x 0.20 = 100 mg
If you'd rather skip the math equations, a modern joint may contain between 60 to 150 mg of THC or more, representing a sharp increase from the approximately 10 mg common in the 1970s. 
The above formulas tell us how much THC is in a joint, but they tell us nothing about how much your body is absorbing from the joint. According to Rae, this number will depend on several factors that differ among individuals. These factors include:

• Composition of the joint, i.e., ratio of THC to CBD
• How much THC is wasted in sidestream smoke which wafts off at the end of a joint without being consumed, unlike mainstream smoke which is what you directly inhale and exhale
• Smoking topography, aka your puff technique
• Bioavailability (the proportion of THC that is able to activate the brain, after being introduced into the body and entering the bloodstream)

Rae elaborated on the key components of mainstream and sidestream smoke, saying, “The ratio of mainstream to sidestream smoke produced by a joint is highly variable because of differences in puff techniques. There are several factors in an individual puff. The volume of smoke per puff is typically about 35 milliliters but can be as big as 55 milliliters (for context, the volume of a shot glass is about 40 milliliters). This volume is determined by the force and amount of time a person inhales.”
Mainstream smoke is also affected by the frequency of inhalations, as Rae said, “Puffing more frequently increases the amount of THC consumed in mainstream smoke, and thus diminishes the amount of THC wasted in sidestream smoke.”
Ultimately, bioavailability is calculated by measuring the THC in the blood, and comparing that to how much THC you started with in the joint. Referencing research published in 2007 in the journal Chemistry & Biodiversity, Rae added, “Bioavailability is highly variable for inhalation: experiments in humans show that it ranges from 2% to 56% depending on the person.”
To illustrate bioavailability, Rae provided the following examples using one gram joints and half gram joints.

• One gram joint = 200 mg THC. On the low end, 2%: 200 x 0.02 = 4 mg. On the high end, 56%: 200 x 0.56 = 112 mg.
  The bioavailable THC in a 1 gram joint with 20% THC is between 4  and 112 milligrams.
  Half gram joint = 100 mg THC. On the low end, 2%: 100 x 0.02 = 2 mg. On the high end, 56%: 100 x 0.56 = 56 mg.
  The bioavailable THC in a half-gram joint with 20% THC is between 2 and 56 milligrams

[1]. In 2018 Michigan legalized the use of marijuana for adults. Since this law took effect, increased availability and use of cannabis have led to an increase in emergency department (ED) visits, as well as complaints about the drug's neuropsychiatric effects [2]. High doses of cannabis can be associated with undesired effects such as paranoia, psychotic-like symptoms, and panic attacks, depending on the content of its principal active constituent, D9-tetrahydrocannabinol (THC), and that of cannabidiol, a constituent with anti-anxiolytic properties [3]. Higher concentrations of THC relative to cannabidiol, as demonstrated in many commercial products, are associated with increased incidence of psychotomimetic symptoms [4,5]. Various studies in adults with acute THC exposure have shown a dose-related reduction in performance at laboratory tasks measuring memory, divided and sustained attention or concentration, reaction time, and tracking and motor function [3,6-9]. Less is known regarding the acute effects of THC in the pediatric population, and much of the

data that currently exist involve retrospective reviews of large, coded data sets [10-12]. The purpose of this study was to describe the prevalence, clinical features, and disposition of cannabis neuropsychiatric toxicity in a community-based study.

 All cannabis‐based and cannabinoid medications should be subjected to the  rigorous scrutiny of the Federal Food and Drug Administration (FDA) regulatory process.  This process provides important protections for patients, making medications available  only when they: 1) are standardized by identity, purity, potency and quality; 2) are accompanied by adequate directions for use in the approved medical indication; and 3)  have risk/benefit profiles that have been defined in well‐controlled clinical trials. 

Detailed Information
. Patients freely share their “medicine” with family and friends, and parents are self-diagnosing, and subsequently dosing their children with high-concentration marijuana products for conditions that may simply not exist (attention-deficit/hyperactivity disorder, anxiety, etc.); or allow their medical cards to expire and continue to grow their own.
The use of marijuana for medical conditions, including pain, needs robust studies, and subsequent products need more regulation and consistency for public consumption. Colorado is an example of the societal effect across a wide spectrum of arenas that comes with rampant cannabis use, particularly in youth use and impaired driving fatalities. The problems Colorado is seeing completely transcend “responsible use” or “marijuana as medicine.” 

PowerPoint Presentation comparing Marijuana & Alcohol,

Pot is becoming more potent, a new study suggests.
In the study, the researchers looked at more than 38,600 samples of illegal marijuana seized by the U.S. Drug Enforcement Administration over 20 years. They found that the level of THC, or tetrahydrocannabinol— marijuana's main psychoactive ingredient — in the marijuana samples rose from about 4 percent in 1995 to about 12 percent in 2014.
Conversely, the level of CBD, or cannabidiol — an ingredient sometimes touted for its potential health benefits — fell from about 0.28 percent in 2001 to less than 0.15 percent in 2014.
But smoking marijuana with high doses of THC may involve a higher risk of negative health effects, such as psychosis orpanic attacks, he said.
"Great article- clearly based on the previous posted comments  marijuana severely impairs a person's ability to understand and process information. Their anecdotal evidence continues to validate scientific statistics. " -No2Pot

See report for details, graphs, data.

The majority of the products we tested were inaccurately labeled," said study author Ryan Vandrey, of The Johns Hopkins University School of Medicine in Baltimore.

Marijuana contains more than 400 chemicals. The primary psychoactive chemical is delta-9- tetrahydrocannabinol (THC) and is the component of marijuana principally responsible for the “high” experienced by pot smokers. Along with the high, users exhibit slower reflexes and decreased coordination. Marijuana also impairs judgment. Slower reaction times, impaired judgment, and problems responding to signals and sounds equal dangerous drivers. No wonder those who drive within three hours of smoking marijuana are twice as likely to be involved in a major car crash.

State of the Science   190 resources
Several problems are associated with marijuana use during pregnancy, because prenatal exposure influences brain development and can result in permanent cognitive impairment. [35] Cognitive deficits resulting from prenatal exposure include inattention; impulsivity; and impairment in learning, memory, and certain aspects of executive functioning. [36] In addition, prenatal exposure to cannabis has been associated with reduction of fetal growth. [37]
For many years, the myth of smoked cannabis being "healthier" or "no worse" than smoked tobacco was perpetuated by pro­marijuana groups. Both smoked cannabis and tobacco contain approximately 4000 chemicals and that these chemicals are essentially identical in both plants.In another recent review, [23] the authors concluded that "smoking of cannabis is not medically recommended due to the potential respiratory tract, dangers of noxious compounds such as polycyclic aromatic hydrocarbons, tar and carbon monoxide."
 
 

The fact is that there is no level of THC in blood above which everyone is impaired, and below which no one is impaired.  This is not because we need more studies; it’s because of human biology. Consider the following three factors.

Marijuana today is "genetically modified," with THC levels that "far surpass the marijuana" of the 1970s.    YES

The number of samples confiscated with a THC concentration greater than 9 percent has increased significantly, from 3.2 percent in 1993 to 21.5 percent of the 1,635 marijuana samples collected in 2007.

There is absolutely no reliable scientific evidence that THC is necessary to synergize the effects of CBD. Instead, there is evidence from preclinical research that THC may be pro-convulsant in sensitive brains; other research indicates that chronic use of THC can impair IQ in adolescents. Physicians are beginning to report instances of THC toxicity in children taking “high CBD” preparations, e.g., high anxiety, increased seizures, insomnia, etc. Until more is known, the most conservative course of action would be to remove THC entirely from a CBD product.