The short-term effects of drinking caffeine are well known. Caffeine is a mild stimulant that makes you feel more alert and makes your heart beat faster. It also raises your body temperature, making your digestive system produce more acid. If you drink large doses of caffeine you may experience high blood pressure, restlessness, headaches, nervousness or insomnia.  

The effects of caffeine are temporary. Caffeine does not accumulate in the body and is excreted after a few hours.

Over the years, caffeine has also been linked to a wide range of health concerns. Overall, caffeine use is considered safe by the International Food Information Council (IFIC). The following is a run-down of some of the recent research in different areas of health concern.

A recent study published in Circulation: Journal of the American Heart Association found markedly elevated blood pressure and increased nervous system activity when occasional coffee drinkers drank a triple espresso, regardless of whether or not it contained caffeine. Surprisingly, people who drank coffee on a regular basis showed increased stimulation of sympathetic nerve pathways – but no increase in blood pressure.

The results suggest that some unknown ingredient or ingredients in coffee – not caffeine – is responsible for cardiovascular activation.

Blood cholesterol can be raised by several drinks daily of boiled unfiltered coffee (such as espresso and French-press style). However, filtered coffee does not contain the oil compounds which appear to raise blood cholesterol.

Over the years, both caffeine and coffee have been linked to certain cancers but the IFIC says that these associations are no longer supported by medical research.

Recently scientists in Germany discovered that coffee beans contain acrylamide – a substance that has been found to cause cancer in animals. In June 2002, the World Health Organization said that acrylamide was a cause of concern but that more research was needed about its effects on humans. However, according to the German Coffee Federation, acrylamide is not present in raw coffee beans but is formed when they are roasted. Researchers are currently experimenting with new ways of producing coffee beans that do not create a build-up of acrylamide.

In another area, caffeine is said to have a positive effect on skin cancer when tested on mice. It works by blocking the genetic changes to cells that are caused by solar rays.

Caffeine may heighten symptoms of fibrocystic breast lumps (a type of benign breast disease) in some women, but no evidence has shown that it increases the risk of breast cancer.

Many studies have found increases in blood pressure related to caffeine consumption. However, some of these studies were not adjusted for other factors such as smoking and alcohol use. The IFIC states that "A number of studies have shown that any temporary rise in blood pressure due to caffeine consumption is less than the elevation produced by normal, daily activities." However, if you have an existing problem with high blood pressure, you may need to keep an eye on your caffeine intake.

A 1997 study found that there is little association between caffeine consumption and heart attacks, while another study in 1998 found that there is only a 2.1% increased risk of stroke for those consuming three cups of caffeinated drinks per day.

In a recent study in the Netherlands it was found that participants drinking seven or more cups of coffee per day were half as likely to develop Type 2 diabetes than those who drank two cups or less per day. However, it is not recommended that you increase your caffeine intake in line with these findings. Research shows that healthy diet, exercise and maintaining a healthy weight can better prevent the onset of Type 2 diabetes.

A recent Harvard study also demonstrated that 6 cups of coffee a day dramatically reduces risk of Type 2 diabetes, particularly in men. But again, if you choose to drink coffee, also eat well, exercise, and maintain a healthy weight as these approaches can prevent the onset of Type 2 diabetes.

Studies of osteoporosis and coffee drinking have yielded varied results, but in general it is thought that moderate caffeine consumption (no more than 250 mg per day) is not an important risk factor for osteoporosis, particularly where women consume a healthy balanced diet. A study of 980 post-menopausal women who drank the equivalent of two cups of caffeinated coffee per day throughout their lifetime was associated with decreased bone density only in those women who did not drink milk on a daily basis.

There is little evidence to suggest that caffeine has an adverse effect for women who are trying to conceive. However, it might increase the risk of miscarriage or a low-weight birth baby. Caffeine may also effect sperm motility in men, which may contribute to fertility issues.

Pregnant women are advised not to consume caffeine as it does reach the fetus and may disrupt the developing baby’s rest periods. The caffeine may disrupt the developing baby's rest periods and it is common for newborns to experience caffeine withdrawals.

Several studies had, for a time, suggested that caffeine consumption increased the risk of Rheumatoid Arthritis. However a 2003 Harvard Medical School study concluded that such a connection was invalid, at least for women.

Using results from the Nurses’ Health Study, researchers concluded that there was no significant association between caffeine consumption from caffeinated or decaffeinated tea or coffee and the risk of Rheumatoid Arthritis in women.

Caffeine can increase your stress levels. In a recent study researchers found that coffee drinkers felt more stressed, their adrenaline levels were higher and they had an increased heart rate.

Some people rely on caffeine to get them through a stressful period, such as exams or a deadline, but if you are feeling very stressed, cutting down or cutting out caffeine may reduce your stress levels more effectively. While moderate caffeine intake (no more than 250 mg per day, or 10-20 oz of brewed coffee) is safe, a stressful lifestyle is not healthy over the long term, so any steps to alleviate stress are beneficial.