Key takeaways
- Taking Synthroid with certain medications such as phosphate binders, bile acid sequestrants, proton pump inhibitors, and antacids may decrease its effectiveness, making proper timing of doses essential.
- Synthroid can aggravate specific medical conditions, including untreated adrenal gland disorders, heart disease, diabetes, and bone-related conditions, so close monitoring and possible dose adjustments are important.
- To lower the risk of interactions, always inform your doctor about every medication, supplement, and health condition you have before beginning Synthroid therapy.
Synthroid (levothyroxine) is a prescription medication used in adults and children to treat:
- hypothyroidism
- a certain kind of thyroid cancer
The active ingredient in Synthroid is levothyroxine. (An active ingredient is the component that makes a medication effective.)
As with many medications, Synthroid (levothyroxine) can interact with other substances. If you’re wondering about thyroid medication timing—especially questions like How long after taking levothyroxine can I take prednisone—understanding potential drug interactions is essential for safe and effective treatment.
Interactions happen when one substance alters how another works in your body. For instance, alcohol, another prescription drug, or a dietary supplement may change how levothyroxine is absorbed or metabolized. Certain medical conditions can also influence how Synthroid affects you.
Continue reading to explore possible Synthroid interactions and how to manage them safely. For additional background on this medication, including detailed uses and side effects, see this Synthroid overview article.
When should I avoid Synthroid?
Some health conditions or personal factors may increase your risk of complications with Synthroid. In these cases, your doctor may decide not to prescribe it. These are called contraindications. Below are known contraindications for Synthroid.
If you have untreated adrenal gland problems: Synthroid can influence the hormones produced by your adrenal glands. If these hormone levels are already low, taking Synthroid could make the condition worse. Before starting treatment, tell your doctor if you have untreated adrenal gland problems. They will likely address this issue before prescribing Synthroid.
If you’ve had an allergic reaction: If you’ve experienced an allergic reaction to Synthroid or any of its ingredients, your doctor will likely avoid prescribing it again. Taking the medication could trigger another allergic response. Ask your doctor about alternative treatment options.
Before beginning Synthroid, review your full medical history with your doctor to confirm that it’s a safe choice for you.
Does Synthroid interact with alcohol?
No direct interactions between Synthroid and alcohol are currently known. However, alcohol may intensify certain side effects associated with Synthroid, including nausea or headache.
If you drink alcohol, ask your doctor how much—if any—is appropriate while taking Synthroid.
Does Synthroid interact with other drugs?
Before starting Synthroid, provide your doctor and pharmacist with a complete list of prescription medications, over-the-counter drugs, and supplements you use. This proactive step can help prevent unwanted interactions.
If you have concerns about drug combinations—such as corticosteroids like prednisone—your healthcare provider can guide you. For example, patients often ask, How long after taking levothyroxine can I take prednisone? Timing and monitoring may be important. You can also read more about this topic here: Can I take levothyroxine and prednisone together and Can you take prednisone with thyroid Medicine.
The table below outlines medications that may interact with Synthroid. Note that this is not a complete list. Additional details appear in the “Drug interactions explained” section.
| Drug group or drug name | Drug examples | What can happen |
| phosphate binders | • ferrous sulfate (iron)• sevelamer (Renvela)• lanthanum (Fosrenol) | can make Synthroid less effective than usual |
| bile acid sequestrants | • colesevelam (Welchol)• cholestyramine (Prevalite)• colestipol (Colestid) | can make Synthroid less effective than usual |
| proton pump inhibitors | • esomeprazole (Nexium)• omeprazole (Prilosec)• pantoprazole (Protonix) | can make Synthroid less effective than usual |
| antacids or gas treatments | • calcium carbonate (Tums)• aluminum hydroxide/magnesium hydroxide (Maalox)• simethicone (Gas-X) | can make Synthroid less effective than usual |
| certain birth control pills taken by mouth | • ethinyl estradiol/norethindrone (Junel, Balziva)• desogestrel/ethinyl estradiol (Enskyce, Kariva)• drospirenone/ethinyl estradiol (Loryna, Yaz) | can make Synthroid less effective than usual |
| certain seizure medications | • carbamazepine (Tegretol)• phenobarbital• phenytoin (Dilantin) | can make Synthroid less effective than usual |
| nonsteroidal anti-inflammatory drugs | • ibuprofen (Advil, Motrin)• naproxen (Aleve)• meloxicam (Mobic) | can make Synthroid less effective than usual |
| tyrosine-kinase blockers | • imatinib (Gleevec)• nilotinib (Tasigna) | can make Synthroid less effective than usual |
| beta-blockers | • propranolol (Inderal XL)• metoprolol (Lopressor, Toprol XL)• bisoprolol (Bystolic) | can make beta-blockers less effective than usual |
| diabetes medications | • glipizide (Glucotrol XL)• insulin lispro (Humalog)• insulin glargine (Lantus)• metformin (Fortamet) | can make diabetes medications less effective than usual |
| corticosteroids | • prednisone (Rayos)• dexamethasone• methylprednisolone (Medrol) | can make Synthroid less effective than usual and can increase the risk of side effects from Synthroid |
| stimulants taken for weight loss | • phentermine (Adipex-P, Lomaira) | can increase the risk of side effects from Synthroid |
| androgens or anabolic steroids | • testosterone (Androderm, Testim, Xyosted) | can increase the risk of side effects from Synthroid |
| tricyclic antidepressants | • amitriptyline• nortriptyline (Pamelor) | can increase the risk of side effects from Synthroid and antidepressants |
| orlistat (Alli, Xenical) | — | can make Synthroid less effective than usual |
| sodium polystyrene sulfonate (Kalexate) | — | can make Synthroid less effective than usual |
| sucralfate (Carafate) | — | can make Synthroid less effective than usual |
| methadone (Methadose) | — | can make Synthroid less effective than usual |
| 5-fluorouracil (Carac, Efudex) | — | can make Synthroid less effective than usual |
| mitotane (Lysodren) | — | can make Synthroid less effective than usual |
| tamoxifen (Soltamo) | — | can make Synthroid less effective than usual |
| furosemide (Lasix) | — | can make Synthroid less effective than usual |
| heparin | — | can make Synthroid less effective than usual |
| sertraline (Zoloft) | — | can make Synthroid less effective than usual |
| rifampin (Rimactane) | — | can make Synthroid less effective than usual |
| amiodarone (Pacerone) | — | can make Synthroid less effective than usual and can increase the risk of side effects from Synthroid |
| digoxin (Lanoxin) | — | can make digoxin less effective than usual |
| warfarin (Jantoven) | — | can increase the risk of side effects from warfarin |
| ketamine (Ketalar) | — | can increase the risk of side effects from ketamine |
Drug interactions explained
Below is a closer look at select Synthroid interactions and how they may affect your thyroid hormone management.
Interaction with birth control pills
Birth control pills containing estrogen may interact with Synthroid.
What could happen
Estrogen-containing birth control pills can increase levels of thyroid-binding globulin (TBG) in the bloodstream. Because TBG binds to Synthroid, higher levels may reduce the amount of active medication available, decreasing its effectiveness.
What you can do
If you use birth control pills, inform your doctor before starting Synthroid. More frequent thyroid function monitoring may be needed, and your dosage may be adjusted accordingly.
Interaction with antacids
Antacids used for heartburn relief may interfere with Synthroid absorption.
What could happen
Antacids can alter stomach acidity. Since levothyroxine requires a specific acidic environment for proper absorption, reduced acidity may limit how much of the drug enters your bloodstream.
What you can do
Take antacids at least 4 hours before or after your Synthroid dose. Also notify your doctor if you use these products regularly so thyroid levels can be monitored appropriately.
How can I prevent interactions?
There are several practical steps you can take to reduce the risk of Synthroid interactions and optimize thyroid treatment outcomes.
- Share a complete list of medications, vitamins, and herbal supplements with your healthcare provider.
- Discuss alcohol or cannabis use openly.
- Ask specific timing questions, such as How long after taking levothyroxine can I take prednisone, especially if you are prescribed corticosteroids.
- Take Synthroid on an empty stomach with plain water, at least 30 minutes to 1 hour before eating.
- Separate calcium, iron, magnesium, and multivitamins from Synthroid by at least 4 hours.
Always review Synthroid’s official prescribing information and follow your doctor’s instructions closely. Taking the medication exactly as directed significantly lowers the risk of complications.
Disclaimer: HealthEH has made every effort to ensure that the information presented here is accurate, comprehensive, and current. However, this content should not replace professional medical advice. Always consult a qualified healthcare provider before starting or adjusting any medication. Drug information may change and does not cover all possible uses, precautions, warnings, drug interactions, or adverse effects. The absence of specific warnings does not mean a medication is safe or appropriate for every individual.
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