Reviewed and updated: August 2026
Immediate cannabis effects can include euphoria, dizziness, increased heart rate, anxiety, panic, paranoia, impaired memory and attention, dry mouth, red eyes and drowsiness. Effects vary with dose, THC level, route of use, tolerance, health and other substances. Higher doses and higher-THC products generally increase the risk of unpleasant effects.
Why does cannabis cause red eyes?
THC can widen blood vessels and increase blood flow in the eyes, making surface vessels appear more visible. Smoke and dry air may add irritation. Red eyes are common, but severe eye pain, sudden vision changes or swelling require medical assessment.
Why does cannabis cause dry mouth?
Cannabinoid signalling can reduce saliva production, causing “cottonmouth.” Sip water, avoid combining cannabis with alcohol and consider sugar-free gum. Persistent dry mouth matters because saliva helps protect teeth and gums.
Can cannabis increase heart rate?
Yes. Increased or rapid heart rate is a recognized immediate effect, particularly with THC. People with heart or blood-vessel disease should speak with a healthcare professional before using cannabis. Seek urgent care for chest pain, fainting, severe shortness of breath or a sustained rapid or irregular heartbeat.
Can cannabis make you tired?
Yes. Drowsiness or fatigue can occur during intoxication and may linger into the next day. Never drive or operate equipment while impaired.
Can cannabis cause anxiety, paranoia or memory problems?
THC can cause anxiety, panic, paranoia, disorganized thinking and short-term difficulty with attention or memory. Rare serious reactions include hallucinations and psychotic symptoms. Risk may rise with high doses, high-THC products and a personal or family vulnerability to psychosis.
What happens if someone consumes too much?
Cannabis poisoning may cause severe anxiety, panic, vomiting, confusion, loss of coordination, hallucinations or other distressing symptoms. Fatal poisoning from cannabis alone is considered extremely unlikely, but injuries and medical emergencies can still occur.
Call 911 for severe or life-threatening symptoms. Otherwise, contact a provincial poison centre for advice.
Longer-term risks
Frequent long-term use can increase the risk of cannabis dependence or addiction, mental-health problems and respiratory harm when smoked. Contrary to a common myth, cannabis can cause physical dependence and withdrawal.
Primary sources: Health Canada, “Cannabis and your health”; Health Canada consumer information; Health Canada, “Addiction to cannabis.”
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