For many of us, the day doesn’t start until we’ve had our first cup of coffee. It’s comforting, energising, and one of the most widely consumed beverages in the world. But while your morning brew might feel harmless, it can interact with certain medicines in ways that reduce their effectiveness – or increase the risk of side-effects.

From common cold tablets to antidepressants, caffeine’s impact on the body goes far beyond a quick energy boost. Tea also contains caffeine but not in the same concentrations as coffee, and doesn’t seem to affect people in the same way. Here’s what you should know about how coffee can interfere with your medications – and how to stay safe.

Cold and flu medicines

Caffeine is a stimulant, which means it speeds up the central nervous system. Pseudoephedrine, a decongestant found in cold and flu remedies such as Sudafed, is also a stimulant. When taken together, the effects can be amplified – potentially leading to jitters or restlessness, headaches, fast heart rate and insomnia.

Many cold medications already contain added caffeine, increasing these risks further. Some studies also suggest that combining caffeine with pseudoephedrine can raise blood sugar and body temperature – particularly important for people with diabetes.

Stimulant effects are also a concern when combining caffeine with ADHD medications such as amphetamines, or with asthma drugs such as theophylline, which shares a similar chemical structure to caffeine. Using them together may increase the risk of side-effects such as a rapid heartbeat and sleep disruption.

2. Thyroid medication

Levothyroxine, the standard treatment for an underactive thyroid, is highly sensitive to timing – and your morning coffee can get in the way. Studies show that drinking coffee too soon after taking levothyroxine can reduce its absorption by up to 50%.

Caffeine speeds up gut motility (the movement of food and waste through the digestive tract), giving the drug less time to be absorbed – and may also bind to it in the stomach, making it harder for the body to take in. These effects reduce the drug’s bioavailability, meaning less of it reaches your bloodstream where it’s needed. This interaction is more common with tablet forms of levothyroxine, and less likely with liquid formulations.

If absorption is impaired, symptoms of hypothyroidism – including fatigue, weight gain and constipation – can return, even if you’re taking your medicine correctly.

The same timing rule applies to a class of osteoporosis medications called bisphosphonates, including alendronate and risedronate, which also require an empty stomach and around 30-60 minutes before food or drink is taken.

3. Antidepressants and antipsychotics

The interaction between caffeine and mental health medications can be more complex.

Selective serotonin reuptake inhibitors (SSRIs), such as sertraline and citalopram, are a type of antidepressant medication widely used to treat depression, anxiety and other psychiatric conditions. Lab studies suggest caffeine can bind to these drugs in the stomach, reducing absorption and potentially making them less effective.

Tricyclic antidepressants (TCAs), such as amitriptyline and imipramine, are a class of older antidepressants that work by affecting the levels of neurotransmitters in the brain. They were among the first antidepressants developed and are less commonly used today, compared with newer antidepressants such as SSRIs, due to their potential for more side-effects and higher risk of overdose.

TCAs are broken down by the liver enzyme CYP1A2, which also metabolises caffeine. The competition between the two can slow drug breakdown, increasing side-effects, or delay caffeine clearance, making you feel jittery or wired longer than usual.

Clozapine, an antipsychotic, is also processed by CYP1A2. One study showed that drinking two-to-three cups of coffee could increase blood levels of clozapine by up to 97%, potentially increasing risks such as drowsiness, confusion, or more serious complications.

4. Painkillers

Some over-the-counter painkillers, such as those containing aspirin or paracetamol, include added caffeine. Coffee can speed up how quickly these drugs are absorbed by accelerating how fast the stomach empties and making the stomach more acidic, which improves absorption for some medications such as aspirin.

While this may help painkillers work faster, it could also raise the risk of side-effects like stomach irritation or bleeding, especially when combined with other sources of caffeine. Though no serious cases have been reported, caution is still advised.

5. Heart medications

Caffeine can temporarily raise blood pressure and heart rate, typically lasting three-to-four hours after consumption. For people taking blood pressure medication or drugs that control irregular heart rhythms (arrhythmias), this may counteract the intended effects of the medication.

This doesn’t mean people with heart conditions must avoid coffee altogether – but they should monitor how it affects their symptoms, and consider limiting intake or switching to decaf if needed.