One of my birthday presents this year was a coupon for a free coffee, possibly given to me out of pity as I stumbled into Starbucks for my regular fix after an hour’s snooze in a 9am lecture. As we launch into the second half of the academic year, a large number of the student population will already be stocking up on the Pro Plus for future exam revision.

For others, early-hour stints in the library will be the norm in lives fuelled by an excessive coffee intake. As the world’s best-loved psychoactive substance, 90% of the adult population regularly doses-up on caffeine. But how much do we actually know of its global and long-term biological effects?

Caffeine has been exploited by humankind as a stimulatory drug since as early as the ninth century (although, according to legend, goats were the first to experience its effects…) This stimulatory effect – increased alertness and attention – is the driving force for its popularity, and presumably the massive increase in its consumption during Easter term. It works by antagonising adenosine receptors in the brain, reducing the obstruction on thought processes and coordination usually brought about by these receptors. Unfortunately, as with all drugs, along come those annoying side effects. The adenosine antagonism could potentially cause dependency in a way similar to illegal psychoactive drugs, stimulating dopamine reward pathways in the brain.

Both psychological and physiological addiction can result from excessive caffeine intake, and have been observed in people consuming as little as 100mg of caffeine a day – around half a cup of coffee. If your May Week has ever been followed by headache, muscle pain and stiffness, lethargy, nausea, vomiting, depression or irritability, you may have been experiencing withdrawal symptoms associated with caffeine dependence if you’ve substantially reduced its intake.

The good news is that not everyone may be susceptible to such dependence; certain genes have been linked to the likelihood of experiencing withdrawal symptoms. Want to know if you have the gene? Maybe switch a parent’s coffee to decaff on the sly next time you’re home, and observe.

The second issue is that adenosine receptors are found all over the body, not just in your central nervous system. Caffeine isn’t selective about which part of the body it acts on, and so a whole host of other effects come into play. The queue for the loo in Starbucks is telling: there is some evidence that caffeine is diuretic and acts to increase fluid loss via the kidneys to produce more urine; in some people drinking coffee is quickly followed by an urge to urinate. High doses can contribute to insomnia, anxiety and loss of fine muscle control – in other words, exactly the things you don’t want during exam period.

However, it’s not all bad news; long-term caffeine use appears to have the potential to reduce the likelihood of several serious illnesses, including type-2 diabetes and cancer. It also seems to increase your metabolic rate – great if you’re trying to lose some flab.

So maybe, this exam term, just don’t pop so much pro-plus. Sure, you’ll be able to stay up wide-eyed all night in the library. But you may end up enduring your exam tightly cross-legged, scribbling down illegible words with shaky hands after a sleepless night. Trust me, I speak from experience, just have a couple less coffees a day and go to bloody bed.