Colesevelam and Diltiazem: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Diltiazem
Colesevelam
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Decreased diltiazem bioavailability
Interaction Deep Dive
Concurrent use of diltiazem and colesevelam may result in decreased bioavailability of diltiazem. Concurrent administration of colestipol (a bile acid sequestrant similar to colesevelam) with diltiazem significantly reduced the bioavailability of diltiazem and its active metabolite desacetyldiltiazem, in a crossover study of healthy subjects (n=12) given both immediate-release and sustained-release diltiazem formulations2. If co-treatment with colesevelam and diltiazem is necessary, patients should take diltiazem at least 4 hours prior to colesevelam. Alternatively, patients should be monitored for an adequate response to diltiazem therapy and the diltiazem dose should be adjusted appropriately 1.
Why it happens (mechanism)
Reduced absorption of diltiazem by nonspecific binding with colesevelam and increased gastrointestinal transit time
Literature reports
1 report — tap to read
a) Concurrent administration of diltiazem with colestipol (a bile acid sequestrant similar to colesevelam) significantly reduced the bioavailability of diltiazem and its active metabolite desacetyldiltiazem. In a 2-way-crossover design study, healthy subjects (n=12) received single oral doses of sustained-release (SR) or immediate-release (IR) diltiazem 120 milligrams (mg) on study days 1 and 3, with a concomitant dose of colestipol 15 grams in the morning and evening of days 2 and 3. Following a 7-day washout period, subjects then crossed over to the opposite study arm. On study days 1 and 3, serial blood samples were analyzed for diltiazem and desacetyldiltiazem pharmacokinetics pretreatment and for 24 hours after treatment. For IR-diltiazem, concomitant treatment with colestipol significantly reduced diltiazem peak concentration (Cmax) and area under the 24-hour concentration-time curve (AUC (0-24)) (by 33% and 27%, respectively; p=0.0032 and p=0.0001, respectively), and significantly increased the time to maximum diltiazem concentration (by 58%, p=0.0013). For SR-diltiazem, concurrent treatment with colestipol significantly reduced diltiazem Cmax and AUC (0-24) (by 36% and 23%, respectively; p=0.0014 and p=0.0001, respectively) and increased the time to maximum diltiazem concentration (by 30%, p=0.0070). Similar results were observed with regard to desacetyldiltiazem pharmacokinetics for both diltiazem formulations 2.
Common questions
Can I take Colesevelam and Diltiazem together?
Decreased diltiazem bioavailability Always confirm with your pharmacist or prescriber before making any change.
How serious is the Colesevelam and Diltiazem interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Colesevelam or Diltiazem need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (2)
- Product Information: WELCHOL oral tablets, colesevelam hcl oral tablets. Daiichi Sankyo Inc, Parsippany, NJ, 2008. DailyMed
- Turner SW, Jungbluth GL, & Knuth DW: Effect of concomitant colestipol hydrochloride administration on the bioavailability of diltiazem from immediate- and sustained-release formulations. Biopharm Drug Dispos 2002; 23:369-377. PubMed
Keep reading about Diltiazem
Keep reading about Colesevelam
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist