It Doesn't Take a Lot of Caffeine to Have an Effect. A big cup of coffee can have more than 300 milligrams (mg) of caffeine. This will provide a definite kick, even for people who drink coffee.

If you rely on caffeine to wake you up and keep you going, you aren't alone. Millions of people rely on caffeine every day to stay alert and improve concentration.

Medically reviewed by Drugs.com. Last updated on Mar 6, 2020.

How much is too much?

• Caffeine is one of the most widely used stimulants, and is generally considered safe at moderate levels of use: 300-400 mg of caffeine per day, or the amount in 3-4 cups of coffee. As with most foods that don't contain essential nutrients, the key to caffeine intake is moderation.
• However, espresso servings are typically very small, with a regular cup or shot being about 1 – 1.75 oz (30 – 50 ml), which contains 53 mg of caffeine. If you take a double shot, the caffeine content is about 125 mg.
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• In moderate doses caffeine has mainly positive effects for most people. But it increases production of cortisol, which can lead to health problems including anxiety, weight gain and heart disease.

Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two 'energy shot' drinks. Keep in mind that the actual caffeine content in beverages varies widely, especially among energy drinks.

Caffeine in powder or liquid form can provide toxic levels of caffeine, the U.S. Food and Drug Administration has cautioned. Just one teaspoon of powdered caffeine is equivalent to about 28 cups of coffee. Such high levels of caffeine can cause serious health problems and possibly death.

Although caffeine use may be safe for adults, it's not a good idea for children. Adolescents and young adults need to be cautioned about excessive caffeine intake and mixing caffeine with alcohol and other drugs.

Women who are pregnant or who are trying to become pregnant and those who are breast-feeding should talk with their doctors about limiting caffeine use to less than 200 mg daily.

Even among adults, heavy caffeine use can cause unpleasant side effects. And caffeine may not be a good choice for people who are highly sensitive to its effects or who take certain medications.

Read on to see if you may need to curb your caffeine routine.

You drink more than 4 cups of coffee a day

You may want to cut back if you're drinking more than 4 cups of caffeinated coffee a day (or the equivalent) and you have side effects such as:

• Headache
• Insomnia
• Nervousness
• Irritability
• Frequent urination or inability to control urination
• Fast heartbeat
• Muscle tremors

Even a little makes you jittery

Some people are more sensitive to caffeine than are others. If you're susceptible to the effects of caffeine, even small amounts may prompt unwanted effects, such as restlessness and sleep problems.

How you react to caffeine may be determined in part by how much caffeine you're used to drinking. People who don't regularly drink caffeine tend to be more sensitive to its effects.

You're not getting enough sleep

Caffeine, even in the afternoon, can interfere with your sleep. Even small amounts of sleep loss can add up and disturb your daytime alertness and performance.

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Using caffeine to mask sleep deprivation can create an unwelcome cycle. For example, you may drink caffeinated beverages because you have trouble staying awake during the day. But the caffeine keeps you from falling asleep at night, shortening the length of time you sleep.

You're taking medications or supplements

Some medications and herbal supplements may interact with caffeine. Examples include:

• Ephedrine. Mixing caffeine with this medication — which is used in decongestants — might increase your risk of high blood pressure, heart attack, stroke or seizure.
• Theophylline. This medication, used to open up bronchial airways, tends to have some caffeine-like effects. So taking it with caffeine might increase the adverse effects of caffeine, such as nausea and heart palpitations.
• Echinacea. This herbal supplement, which is sometimes used to prevent colds or other infections, may increase the concentration of caffeine in your blood and may increase caffeine's unpleasant effects.

Talk to your doctor or pharmacist about whether caffeine might affect your medications.

Curbing your caffeine habit

Whether it's for one of the reasons above or because you want to trim your spending on coffee drinks, cutting back on caffeine can be challenging. An abrupt decrease in caffeine may cause withdrawal symptoms, such as headaches, fatigue, irritability and difficulty focusing on tasks. Fortunately, these symptoms are usually mild and get better after a few days.

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To change your caffeine habit, try these tips:

• Keep tabs. Start paying attention to how much caffeine you're getting from foods and beverages, including energy drinks. Read labels carefully. But remember that your estimate may be a little low because some foods or drinks that contain caffeine don't list it.
• Cut back gradually. For example, drink one fewer can of soda or drink a smaller cup of coffee each day. Or avoid drinking caffeinated beverages late in the day. This will help your body get used to the lower levels of caffeine and lessen potential withdrawal effects.
• Go decaf. Most decaffeinated beverages look and taste much the same as their caffeinated counterparts.
• Shorten the brew time or go herbal. When making tea, brew it for less time. This cuts down on its caffeine content. Or choose herbal teas that don't have caffeine.
• Check the bottle. Some over-the-counter pain relievers contain caffeine. Look for caffeine-free pain relievers instead.

The bottom line

If you're like most adults, caffeine is a part of your daily routine. Usually, it won't pose a health problem. But be mindful of caffeine's possible side effects and be ready to cut back if necessary.

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by Stefanie Petrou Binder, MD EyeWorld Contributing Writer

The more coffee you drink, the higher the concentration of antioxidants reaching the lens, according to researchers. New data indicates that drinking coffee before cataract surgery increases the caffeine levels in the lens capsule in a dose dependent manner. Based on the assertion that caffeine’s antioxidant nature could have a positive effect on the prevention of cataract blindness, this finding may be the news coffee lovers have been waiting for.
Speaking on the effects of caffeine on the eye during a free paper session at the 23rd European Society of Cataract and Refractive Surgeons Winter Meeting, Manuel Ruiss, MSc, presented study results that assessed caffeine concentrations in the eyes of patients who drank coffee just before cataract surgery.¹ He worked on the study with his supervisor, Martin Kronschläger, MD.
“Cataract is still the leading cause of blindness across the globe, with no approved drug to prevent the disease,” Mr. Ruiss said in his presentation. “The crystalline lens is constantly subjected to oxidative stress, for instance via ultraviolet radiation. In cataract, reactive oxygen species are increased, like hydrogen peroxide, whereas antioxidants like glutathione, a primary lenticular antioxidant, are decreased. Experimental animal studies and observational data suggest antioxidants might retard the development of cataract.”

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Studying the effects of coffee

The study included 40 patients (80 eyes) scheduled for bilateral cataract surgery, with a gap of 1 week between surgeries. The patients were examined 1 week prior to surgery and asked to abstain from caffeine until the second eye surgery, encompassing 2 weeks of no caffeine. There were no statistically significant differences among age, weight, and usual coffee consumption in the study patients. Patients drank an average of 1.6–2.3 cups of coffee/day. The mean patient age was between 72 and 78 years, and their weight was between 77 and 80 kilos.
Shortly before the second cataract surgery, the patients were asked to consume coffee and were randomized into different groups: 0 mg caffeine (n=10), 60 mg caffeine (n=10), 120 mg caffeine (n=10), and 180 mg caffeine (n=10). The time between coffee consumption and surgery was 144–191 minutes (2–3 hours).
After cataract surgery with capsulorhexis, the lens capsule and lens epithelial cells were collected and sent to a lab for a gas chromatography-mass spectrometry analysis of caffeine concentration. Of the 80 eyes of 40 patients that were included, seven patients (14 eyes) were excluded due to protocol noncompliance. The results showed that coffee intake before cataract surgery increased the caffeine levels in the lens capsule in a dose dependent fashion.

Past caffeine studies

Past studies have examined the antioxidant effect of caffeine and its ability to inhibit oxidative damage. In one such study, caffeine was thought to be an important antioxidant, being a scavenger of free radicals. The study likened the general antioxidant ability of caffeine to that of the established biological antioxidant glutathione and significantly higher than ascorbic acid.² Another study from the literature demonstrated the protection factor (PF) of caffeine against UV radiation to be higher than the PF of both vitamin C and vitamin E.³
“Up until now there has been some evidence that caffeine or antioxidants might play a protective role for lens epithelial cells and a preventive role for cataract, however, no one has shown if caffeine can protect the lens and the lens capsule through peroral intake. Epidemiological findings show that caffeine has had a positive effect on the prevention of cataract blindness,” Mr. Ruiss said.
The incidence of cataract blindness in humans was significantly lower in groups consuming higher amounts of coffee in comparison to those groups with lower coffee intake, according to a study that credited both its antioxidant and bioenergetic effects on the lens. The study assessed per capita coffee consumption in kg/year as obtained from the Food and Agriculture Organization of the United Nations and from other sources.⁴ The dietary total antioxidant capacity, including coffee, of middle-aged and elderly women was inversely associated with the risk of age-related cataract, according to yet another study on the subject. The Swedish population-based study found that coffee contributed 15% of total antioxidant capacity along with fruit and vegetables (44%) and whole grains (17%).⁵
“Caffeine accumulates in the lens capsule and epithelial cells after oral intake of coffee, making it a potent candidate to delay the onset of cataract,” Mr. Ruiss said.
About the doctor
Manuel Ruiss, MSc
Vienna Institute for Research
in Ocular Surgery
Hanusch Hospital
Vienna, Austria

References
1. Kronschläger M, et al. Pharmacokinetics of caffeine in the lens capsule/epithelium after peroral intake: a pilot randomized controlled study. Invest Ophthalmol Vis Sci. 2018;59:1855–1860.
2. Devasagayam TP, et al. Caffeine as an antioxidant: inhibition of lipid peroxidation induced by reactive oxygen species. Biochim Biophys Acta. 1996;1282:63–70.
3. Kronschläger M, et al. Caffeine eye drops protect against UV-B cataract. Exp Eye Res. 2013;113:26–31.
4. Varma SD. Effect of coffee (caffeine) against human cataract blindness. Clin Ophthalmol. 2016;10:213–20.
5. Rautiainen S, et al. Total antioxidant capacity of the diet and risk of age-related cataract: a population-based prospective cohort of women. JAMA Ophthalmol. 2014;132:247–52.

Relevant financial interests
Ruiss: None