Caffeine is a drug used world round for energy, mental alertness and even weight loss. It’s found in coffee, tea, energy drinks, and more. Caffeine is also addictive and can be extremely dangerous in high amounts, for the average person who may only drink up to 3 cups a coffee per day, caffeine is usually safe. 

Side effects:

The list of side effects includes but isn’t limited to

Headache 

Dizziness

Nausea 

Diarrhea 

Lightheadedness 

Rapid heartbeat

Anxiety

Sleeplessness

Agitation

Black, tarry stools

Blood in the stools or urine

Bruising

Burning feeling or tenderness in the stomach

Chills

Coma

Confusion

Coughing or vomiting blood

Decreased urine output

Depression

Fainting

Fever

Hostility

Indigestion

Irritability

Lethargy

Muscle twitching

Persistent bleeding

Rapid, shallow breathing

Rapid weight gain

Rash

Seizures

Severe stomach pain

Stomach upset

Stupor

Swelling of the face, ankles, or hands

Trouble breathing

Unusual tiredness or weakness

Vomiting

Vomiting of blood or material that looks like coffee grounds

Weak bones

https://www.drugs.com/sfx/caffeine-side-effects.html

https://www.webmd.com/vitamins/ai/ingredientmono-979/caffeine