Many factors go into weed tolerance. People have different sensitivities to drugs based on an individual’s rate of metabolism that involve a variety of factors, including:

Age
Physiology
Genetics
Diseases

How Does Weed Tolerance Work?

One study described the process like this:

“Tolerance at all levels of complexity in the brain involves ‘learning' in the sense of the acquisition of compensatory adaptations to the consequences of the presence of a drug-produced disturbance in function.”

In other words, the body is chugging along and then something disturbs that movement. The body has a reaction, over time, where it learns, or adapts, to the disturbance — it compensates in-order to get back its sense of equilibrium. In this case, the disturbance is THC in cannabis. Long-term exposure to THC causes a reduction in the number and signaling efficiency of cannabinoid receptors, called CB1, in the brain, and these changes parallel the development of tolerance to typical cannabinoid effects, such as impaired memory, according to a 2012 study in Nature on rodents. The unknown is whether these reductions translate to a decrease in cannabis sensitivity or just numbers of CB1 receptors. Regardless, after around four weeks cannabis abstinence, the study demonstrated that CB1 receptor density returned to normal levels.

Cannabis Withdrawal Syndrome (CWS)

Trying to quit or reduce cannabis consumption after increasing your weed tolerance could cause cannabis withdrawal syndrome (CWS). According to the fifth addition of “Diagnostic and Statistical Manual of Mental Disorders (DSM-5),” CWS is a host of cannabis use disorders. International Classification of Diseases says CWS is developed from frequent, heavy, or prolonged cannabis use and occurs as a result of the absence of THC in the system. There is an increasing awareness of the existence of the cannabis withdrawal syndrome (CWS) and its increasing emergence in the last 20 years likely due to the increasing psychotropic potency of marijuana originating from the breeding of strains with high THC (10%–18.5%) and low CBD (<0.15%). This is especially true in high-income countries.

Symptoms

According to the DSM-5, cannabis withdrawal syndrome is classified by at least three of the following symptoms within a week after THC sobriety:

Irritability or hostility
Nervousness or anxiety
Poor sleep
loss of appetite
Restlessness
Feelings of depression
Shakiness or tremors
Sweating
Fever
Headaches

In 2019, a peer reviewed study found that cannabis withdrawal symptoms typically start within 24 hours after abrupt discontinuation of cannabis use and typically subsides within one to two weeks.

Causes

Cannabis withdrawal syndrome is caused by frequent, heavy, or prolonged cannabis use. That being said, there are a few factors which affect the severity of withdrawal symptoms including:

Frequency of cannabis use before stopping
Biological sex
Genetic and environmental factors

Treatment

When it comes to treatment or even abstinence, there are no standardized protocols. What is recognized is that treatment usually involves an outpatient setting due to the mild to moderate levels of these withdrawal symptoms.

Other Potential Factors

In March 2019, researchers from the University of California San Diego and Washington State University, conducted a study involving over 2900 subjects where cannabis users reported subjective experiences when having CWS. The goals of the study were to determine whether the side effects of CSW changed due to participants’ age and the reason for use. The report found older people:

And medical users experience CWS differently than younger users.
Were less likely than younger individuals to report being forgetful, difficulty concentrating, making decisions, and finding words.
Were less affected by cognitive effects such as anxiety, paranoia, apath, and decreased motivation.

“Collectively, the results reported...indicate that medically compromised and older individuals (who may have more health concerns) may not experience the same neurocognitive consequences of cannabis as recreational users,” stated the report.

Weed Withdrawal Studies

Of the 2905 cannabis users in the University of California San Diego and Washington State University study,

“35.2% of the total sample reported that withdrawal symptoms were not applicable to them (either because they have never discontinued cannabis use for ‡72 h or because they simply have not experienced any withdrawal effects).”

For those those who reported withdrawal effects, older consumers were less likely to report:

Irritability
Insomnia
Interrupted sleep
Anxiety
Loss of appetite

In comparison to recreational consumers, medical/mixed users were significantly more likely to report:

Anxiety
Loss of productivity
Loss of appetite

The study is careful to note that the latter symptoms "may reflect true withdrawal or could be confounded by a rebound effect (return of medical symptoms treated by cannabis).”