Tizanidine has two absolute contraindications: it should never be combined with ciprofloxacin or fluvoxamine. Beyond those, a long list of medications, substances, and even foods can change how tizanidine works in your body, sometimes dangerously. The drug is broken down almost entirely by a single liver enzyme called CYP1A2, which makes it unusually vulnerable to interactions with anything that slows that enzyme down.
Ciprofloxacin and Fluvoxamine Are Off-Limits
These two drugs are the most dangerous combinations with tizanidine, and the FDA labels both as contraindicated, meaning they should never be taken together under any circumstances.
Ciprofloxacin, a common antibiotic in the fluoroquinolone family, powerfully blocks the liver enzyme responsible for clearing tizanidine from your body. In a study of healthy volunteers, taking a single 4 mg dose of tizanidine alongside ciprofloxacin increased the drug’s peak blood level by 7 times and overall exposure by 10 times. That’s the equivalent of your body processing a dose many times larger than what you actually swallowed. The result was significant drops in blood pressure, heavy drowsiness, and impaired coordination.
Fluvoxamine, an antidepressant sometimes prescribed for OCD and anxiety, is even more potent. The same single 4 mg tizanidine dose combined with fluvoxamine produced a 12-fold increase in peak blood level and a 33-fold increase in overall drug exposure. The drug also stayed in the body three times longer than normal. If you need an antibiotic or an antidepressant while taking tizanidine, your prescriber needs to choose an alternative that doesn’t block CYP1A2.
Other CYP1A2 Inhibitors to Watch For
Ciprofloxacin and fluvoxamine get the most attention because they’re the strongest CYP1A2 blockers, but other medications also slow this enzyme. Some examples include cimetidine (an older heartburn drug), certain other fluoroquinolone antibiotics, and some antipsychotic medications. Even curcumin, the active compound in turmeric supplements, appears on interaction lists. Anything that inhibits CYP1A2 can raise tizanidine levels in your blood, increasing the risk of dangerously low blood pressure, extreme sedation, and in some cases, heart rhythm changes. Research has shown that tizanidine can prolong the QT interval (a measure of heart electrical activity), and that risk climbs when drug elimination is impaired by CYP1A2 inhibition.
Alcohol Raises Drug Levels
Alcohol increases the amount of tizanidine circulating in your bloodstream by roughly 20%, with peak levels climbing about 15%. That alone raises the risk of side effects. On top of that, alcohol and tizanidine both suppress the central nervous system, so their sedative effects stack. The combination can cause profound drowsiness, slowed reflexes, and impaired judgment beyond what either substance would cause alone.
Sedatives, Opioids, and Sleep Aids
Tizanidine’s sedative effect is additive with a wide range of other central nervous system depressants. These include benzodiazepines (like diazepam and clonazepam), opioid painkillers (like codeine), tricyclic antidepressants (like doxepin and clomipramine), other muscle relaxants (like cyclobenzaprine and baclofen), and cannabis-based products including CBD. Combining any of these with tizanidine increases the risk of excessive sedation, which in severe cases can slow breathing or make it dangerous to drive or operate machinery.
Over-the-counter sleep aids and antihistamines that cause drowsiness, such as cetirizine, deserve the same caution. If you take anything to help you sleep or calm anxiety, check whether it has sedating properties before combining it with tizanidine.
Blood Pressure Medications
Tizanidine lowers blood pressure on its own. Combining it with medications that also reduce blood pressure can cause dangerously low readings. A published case report documented profound, life-threatening hypotension in a patient taking tizanidine alongside lisinopril, a common ACE inhibitor used for high blood pressure. The episodes correlated directly with tizanidine doses and became more severe when lisinopril was also on board. If you take any blood pressure medication, your prescriber should be aware you’re also on tizanidine so they can monitor for symptoms like dizziness, lightheadedness, or fainting.
Oral Contraceptives
Hormonal birth control pills reduce the clearance of tizanidine by approximately 50%. That means the drug lingers in your body roughly twice as long as it would otherwise, raising both blood levels and the chance of side effects like drowsiness, dry mouth, and low blood pressure. If you take oral contraceptives, you may be more sensitive to tizanidine than typical dosing guidelines suggest, and your dose may need to be lower.
Food Changes How Tizanidine Absorbs
Whether you take tizanidine with food or on an empty stomach significantly changes how the drug behaves, and the effect depends on whether you’re taking a tablet or a capsule. Tablets taken with food produce a peak blood level about 23% higher and overall drug exposure about 45% higher compared to taking them on an empty stomach. Capsules are less affected by food (under 20% difference), but food delays the time to peak effect from about 1.4 hours to 3 hours.
The practical takeaway: pick one approach and stick with it. If you switch between taking tizanidine with and without food, you’ll get unpredictable blood levels, which can mean either inadequate muscle relaxation or an unexpected spike in side effects. Similarly, don’t swap between tablet and capsule forms without guidance, since they don’t behave identically.
Kidney Problems Change the Equation
Your kidneys play a significant role in clearing tizanidine. When kidney function drops below a certain threshold (creatinine clearance under 25 mL/min), clearance of the drug falls by more than 50%. Signs that the drug is building up include worsening dry mouth, unusual drowsiness, weakness, and dizziness. People with reduced kidney function typically need lower individual doses rather than less frequent dosing.
Herbal Supplements and OTC Products
Herbal supplements that have sedative properties carry the same stacking risk as prescription sedatives. Valerian, kava, and any “calming” or “sleep support” supplement can amplify tizanidine’s drowsiness. The Mayo Clinic’s interaction database lists a broad range of over-the-counter and supplement products that may interact with tizanidine. Because supplements aren’t regulated with the same rigor as prescription drugs, their potency can vary between brands, making the interaction harder to predict. A good rule of thumb: if a supplement makes you sleepy or claims to promote relaxation, treat it with the same caution you’d give a prescription sedative while you’re on tizanidine.