For millions across the UK, the day doesn't properly begin until that first, vital cup of coffee. It's a cherished ritual, offering comfort and a much-needed energy boost. However, this seemingly harmless habit can pose a significant risk by interfering with a range of common medicines, potentially reducing their effectiveness or amplifying unwanted side effects.
Key Medication Interactions to Be Aware Of
The stimulant effects of caffeine, which go far beyond a simple pick-me-up, can clash with several drug classes. While tea contains caffeine, its lower concentration and different composition mean it doesn't typically cause the same level of interaction. Here are five common types of medication where caution with coffee is advised.
1. Cold Remedies and Other Stimulants
Caffeine is a central nervous system stimulant. This becomes problematic when combined with medicines that have similar properties, such as the decongestant pseudoephedrine, found in products like Sudafed. Taking them together can amplify effects, leading to jitteriness, headaches, a rapid heartbeat, and insomnia.
Many over-the-counter cold and flu tablets already include added caffeine, compounding these risks. Research indicates the combination may also elevate blood sugar and body temperature, a particular concern for individuals with diabetes. Similar caution applies to ADHD medications like amphetamines and certain asthma drugs such as theophylline, which shares a chemical similarity with caffeine.
2. Thyroid and Osteoporosis Treatments
The absorption of levothyroxine, the standard treatment for an underactive thyroid gland, is highly sensitive. Drinking coffee too soon after taking this medication can slash its absorption by up to 50%.
This happens because caffeine accelerates gut motility, reducing the time available for the drug to be absorbed, and may bind to the medication in the stomach. The result is lower bioavailability, meaning less of the drug reaches your bloodstream. Impaired absorption can cause symptoms of hypothyroidism—like fatigue and weight gain—to return.
The same principle applies to bisphosphonate drugs for osteoporosis, including alendronate. These also require being taken on an empty stomach with water, waiting 30 to 60 minutes before any food or drink, including coffee.
3. Mental Health Medications
Interactions with antidepressants and antipsychotics are particularly complex. Laboratory studies suggest caffeine can bind to common SSRI antidepressants like sertraline in the stomach, potentially reducing their absorption and effectiveness.
Older tricyclic antidepressants (TCAs), such as amitriptyline, and the antipsychotic clozapine are broken down by the liver enzyme CYP1A2, which also metabolises caffeine. This creates competition, which can slow the breakdown of the drug, increasing side effects, or delay caffeine clearance, prolonging feelings of jitteriness.
Notably, one study found that drinking two to three cups of coffee could increase blood levels of clozapine by up to 97%, raising the risk of drowsiness, confusion, or more serious complications.
4. Common Painkillers
Some over-the-counter pain relievers containing aspirin or paracetamol include added caffeine. Coffee can speed up the absorption of these drugs by accelerating stomach emptying and increasing acidity. While this might help the medication work faster, it could also elevate the risk of side effects like stomach irritation or bleeding, especially when combined with other caffeine sources.
5. Heart and Blood Pressure Drugs
Caffeine can cause a temporary rise in blood pressure and heart rate for three to four hours after consumption. For individuals taking medication to control hypertension or irregular heart rhythms (arrhythmias), this effect may counteract the intended benefits of their prescription. This doesn't necessarily mean avoiding coffee entirely, but monitoring its impact and considering decaffeinated options is wise.
How to Enjoy Your Coffee Safely
To ensure your daily brew doesn't undermine your health, follow these key steps. Always take levothyroxine and bisphosphonates on an empty stomach with water and wait at least 30 to 60 minutes before having coffee or food.
Exercise caution with stimulant-based cold remedies, ADHD medications, and asthma treatments. If you are prescribed antidepressants, antipsychotics, or heart medications, have a conversation with your GP or pharmacist about your caffeine intake.
Consider reducing your intake or switching to decaf if you experience restlessness, insomnia, or heart palpitations. Everyone metabolises caffeine differently, so pay close attention to how your body responds. If you are ever uncertain about a potential interaction between your medicine and your coffee, a quick chat with a healthcare professional can provide clarity and peace of mind.
This advice is based on insights from Dipa Kamdar, a Senior Lecturer in Pharmacy Practice at Kingston University, originally published by The Conversation on Monday 8 December 2025.
