Marijuana is not unhealthy physically past the age of full brain development.

One of the earliest studies investigating the effect of cannabis on the liver was published in The Lancet in 1971; abnormal liver enzymes and hepatic dysfunction were found in young consumers who were chronic cannabis users, amplified by co-ingestion of alcohol (6). From there, knowledge in this area continued to grow. Figure 1 outlines the principal hepatic metabolism of cannabis by the cytochrome 450 (CYP450) system, a critical step followed by glucuronidation, which facilitates the final fecal and urinary excretions (7,8,9,10).

Marijuana can cloud your senses and judgment. The effects can differ depending on things like how potent your pot was, how you took it, and how much marijuana you’ve used in the past.It might:

• Heighten your senses (colors might seem brighter and sounds might seem louder)
  
• Distort your sense of time
  
• Hurt your motor skills and make driving more dangerous
  
• Lower your inhibitions so you may have risky sex or take other chances
  

About 1 in 10 people who use marijuana will become addicted. That means you can’t stop using it even if it harms your relationships, job, health, or finances. The risk is greater the younger you start marijuana and the more heavily you use it. For instance, the odds of addiction are 1 in 6 if you use pot in your teens. It might be as high as 1 in 2 among those who use it every day.

You could also grow physically dependent on marijuana. Your body could go into withdrawal, leaving you irritable, restless, unable to sleep, and uninterested in eating when you don’t use it.  Learn more about how to spot the signs of marijuana addiction.

It May Impair Your Brain

Marijuana can make it harder for you to focus, learn, and remember things. This seems to be a short-term effect that lasts for 24 hours or longer after you stop smoking.

But using pot heavily, especially in your teen years, may leave more permanent effects. Imaging tests with some -- but not all -- adolescents found that marijuana may physically change their brains. Specifically, they had fewer connections in parts of the brain linked to alertness, learning, and memory, and tests show lower IQ scores in some people.

Your Lungs May Hurt

Marijuana smoke can inflame and irritate your lungs. If you use it regularly, you could have the same breathing problems as someone who smokes cigarettes. That could mean ongoing cough with colored mucus. Your lungs may more easily pick up infections. That’s partly because THC seems to weaken some users’ immune systems.

You May Feel Hungrier

Many people who use marijuana regularly notice that it boosts their appetite. They call this “the munchies.” Some research suggests that might help people with AIDS, cancer, or other illnesses regain weight. Scientists are studying this and whether it’s safe.

It May Harm Your Heart

Marijuana makes your heart work harder. Normally the heart beats about 50 to 70 times a minute. But that can jump to 70 to 120 beats or more per minute for 3 hours after the effects kick in. The added strain plus tar and other chemicals in pot may raise your chance of heart attack or stroke. The danger is even bigger if you’re older or if you already have heart problems.

It Intensifies Alcohol’s Dangers

More than 1 in 10 drinkers say they have used marijuana in the past year. Combining alcohol and marjuana at the same time roughly doubled the odds of drunk driving or legal, professional, or personal problems compared to drinking alone.

Your Newborn Might Be Underweight

Mothers who smoke pot while pregnant face a higher risk of giving birth to underweight or premature babies. But researchers don’t know enough to say if those infants are more likely to grow up to struggle in school, use drugs, or have other problems in life.

Physical health risks

Smoking cannabis can;

• make you wheeze and out of breath
  
• make you cough uncomfortably or painfully
  
• make your asthma worse if you have it
  

There's been less research on it but smoking cannabis is likely to have many of the long term physical health risk as smoking tobacco (even if you don't mix the cannabis with tobacco). So smoking cannabis can also;

• increase the risk of lung cancer
  
• increase your heart rate and affect your blood pressure, which makes it particularly harmful for people with heart disease
  
• reduce your sperm count if you're male, affecting your ability to have children
  
• suppress your ovulation if you’re female, affecting your ability to have children
  
• increase the risk of your baby being born smaller than expected if you smoke it while pregnant
  

Mental health risks

Using cannabis can:

• affect your motivation to do things
  
• impair your memory so you can’t remember things or learn new information
  
• give you mood swings
  
• disturb your sleep and make you depressed
  
• make you anxious, panicky, or even aggressive
  
• make you see or hear things that aren’t there (known as hallucinating or tripping)
  
• cause hours (or days) of anxiety, paranoia and hallucinations, which only settle down if the person stops taking it – and sometimes don’t settle down at all
  
• cause a serious relapse for people with psychotic illnesses like schizophrenia
  
• increase your chances of developing illnesses like schizophrenia, especially if you have a family background of mental illness and you start smoking in your teenage years
  

What is cannabis cut with?

Lots of things. Dealers cut hash with similar-looking substances or heavy materials to increase the weight of the drug and make a bigger profit.
Although not all cannabis is cut, it’s very hard to know when it is or isn’t – so you could be smoking, eating or vaping chemicals from all sorts of unknown substances, including pesticides used when growing the cannabis.
Tobacco is often mixed with cannabis, for making joints or smoking bongs. If you mix cannabis with tobacco you’ll be taking on the same risks you get from smoking tobacco.
These are: addiction to nicotine (the drug in tobacco), coughs, chest infections and in the longer-term, cancer and heart disease.