How Weed Affects Your Eyes and Brain, According to Research

Weed affects your brain and your eyes mainly through THC (tetrahydrocannabinol), its main psychoactive compound, which plugs into the body's endocannabinoid system and changes how your brain processes sensory information. In the short term that means altered perception, memory and coordination; for your eyes, it means redness plus measurable dips in sharpness, contrast and focus while you're high. It touches all five senses, too: sight, hearing, smell, taste and touch.

These effects can vary a lot from person to person depending on dose, strain and individual tolerance. Understanding how weed affects the senses gives you a fuller picture of its impact on the human experience, as well as its potential therapeutic uses.

Side effects of weed on the brain: short term and long term

Weed influences the brain both in the short and the long run, changing how it works and, in young people, how it develops.

Short-term effects

When you consume weed, THC is quickly absorbed into the bloodstream and reaches the brain and other organs. When smoked or vaped, the effects kick in within minutes; with edibles, they usually take 30 to 60 minutes or more, bringing a range of sensory and cognitive changes. These include:

• Altered senses, like colors looking more vivid.
• A warped sense of time.
• Mood swings.
• Trouble with body movements and coordination.
• Difficulty solving problems and thinking clearly.
• Short-term memory loss.
• Possible hallucinations and delusions at high doses.
• A risk of psychosis with regular use of high-potency weed.

Long-term effects of weed on the brain

Weed can also affect brain development, especially when someone starts using it as a teenager. During that critical stage, it may interfere with thinking, memory and other cognitive functions, as well as with the formation of the neural connections those functions depend on.

Studies suggest that heavy, long-term use from adolescence into early adulthood may be linked to a significant drop in IQ.

What do studies say about the effects of smoking weed?

For example, some research has found that people who used weed heavily as teens showed a decline in IQ that persisted even after they quit.

On the other hand, twin studies suggest that this decline could also be explained by shared genetic or environmental factors, not just by cannabis use.

The Adolescent Brain Cognitive Development (ABCD) Study, backed by the US National Institute on Drug Abuse (NIDA), is shedding light on how cannabis and other substances affect the developing teenage brain, giving us a fuller picture of weed's long-term impact.

How does weed affect your eyes?

One of the most obvious effects of using weed is red eyes, caused by THC dilating the blood vessels.

That one's common knowledge, but researchers are still looking into other ways cannabis might affect your vision, like changes in peripheral vision, eye pressure and visual processing.

Does weed affect your vision? What the research says

A study published in Nature's journal Scientific Reports set out to measure these visual effects and compare them with how users perceived them. The research team ran a thorough set of vision tests on 31 cannabis users, before and after smoking.

They also asked participants how they felt their vision had changed after getting high.

The results showed that, after use, several aspects of vision were significantly affected, including visual acuity, contrast sensitivity, 3D vision (stereopsis), focusing ability and sensitivity to glare.

Despite that, 30% of participants said their vision hadn't changed at all, and 65% said it had only gotten slightly worse.

Bottom line: does smoking weed affect your eyesight?

The study's authors stressed the importance of raising awareness about these possible visual effects, since they could pose a risk during everyday tasks like driving.

The fact that users barely notice the drop in vision underlines the need for awareness campaigns about the risks that come with getting high.

How does weed affect motivation and mental health?

Cannabis doesn't just affect how we perceive and think, it can also affect motivation and mental health.

Earlier research suggests that cannabis may affect the brain circuits tied to reward and motivation, which could make people less willing to do school or work tasks and less satisfied with what they achieve.

More studies on weed's side effects

In one recent study, researchers used a reward task inside a brain scanner to see how cannabis affects the brain's response to rewards. During the task, participants watched blue and orange squares and could win money by responding correctly to the orange ones.

Focusing on the ventral striatum, a key region in the brain's reward system, they found only subtle effects of cannabis on how the brain responded to rewards. Worth noting: the participants were moderate users, which suggests the effects could be stronger in heavy, chronic users.

There's also evidence that cannabis use may be linked to mental health problems. Some reports have associated it with anhedonia in teenagers, meaning the inability to feel pleasure.

These findings highlight why it matters to understand how cannabis affects motivation and mental health, especially among regular users, and why more research is needed to properly inform people about the risks.