Key takeaways
- Taking Synthroid with certain medications like phosphate binders, bile acid sequestrants, proton pump inhibitors, and antacids may reduce its effectiveness, so timing its administration is important.
- Synthroid may worsen certain health conditions, such as untreated adrenal gland problems, heart issues, diabetes, and bone conditions, so it requires careful monitoring and dose adjustments.
- To prevent interactions, it’s important to inform your doctor about all medications and supplements you take and your complete health history before starting Synthroid treatment.
Synthroid (levothyroxine) is a prescription drug used to treat the following conditions in adults and children:
- hypothyroidism
- a certain kind of thyroid cancer
The active ingredient in Synthroid is a drug called levothyroxine. (An active ingredient is what makes a medication work.)
Like other drugs, Synthroid (and its active ingredient, levothyroxine) may have interactions.
Some interactions occur because one substance causes another substance to have a different effect than expected. For example, sometimes alcohol, another drug, or a supplement can affect how a drug acts in your body. Interactions can also occur if you have certain health conditions.
Keep reading to learn about the potential interactions of Synthroid. For more information about this drug, including its uses, side effects, and more, see this Synthroid overview article.
Certain health conditions or other factors could raise your risk of harm if you take Synthroid. In such cases, your doctor may not prescribe Synthroid for you. These are known as contraindications. The following list includes contraindications of Synthroid.
If you have untreated adrenal gland problems: Synthroid can affect the amount of hormones that the adrenal glands make. If you have low levels of these hormones, taking Synthroid may worsen your condition. Before taking Synthroid, tell your doctor if you have any untreated adrenal gland problems. They’ll likely treat these problems before prescribing Synthroid.
If you’ve had an allergic reaction: If you’ve had an allergic reaction to Synthroid or any of its ingredients, your doctor likely won’t prescribe Synthroid. This is because taking the drug could cause another allergic reaction. You can ask your doctor about other treatments that may be better options for you.
Before you start taking Synthroid, talk with your doctor if any of the factors above apply to you. They can determine whether Synthroid is safe for you to take.
There are no known interactions between Synthroid and alcohol. But it’s possible for alcohol to worsen certain side effects of Synthroid, such as nausea or headache.
If you drink alcohol, talk with your doctor about how much is safe for you to consume while you’re taking Synthroid.
Before you start taking Synthroid, tell your doctor and pharmacist about any prescription, over-the-counter, or other drugs you take. Sharing this information with them may help prevent possible interactions.
If you have questions about drug interactions that may affect you, talk with your doctor or pharmacist.
The chart below lists some of the drugs that may interact with Synthroid. Keep in mind that this chart does not include all drugs that may interact with Synthroid. For more information about some of these interactions, see 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 |
Learn more about certain drug interactions that can occur with Synthroid.
Interaction with birth control pills
Birth control pills that contain estrogen may interact with Synthroid.
Examples of birth control pills that contain estrogen include:
- ethinyl estradiol/norethindrone (Junel, Balziva)
- desogestrel/ethinyl estradiol (Enskyce, Kariva)
- drospirenone/ethinyl estradiol (Loryna, Yaz)
What could happen
Taking Synthroid with certain forms of birth control pills may make Synthroid less effective. Birth control pills that contain estrogen can raise the level of a protein called thyroid-binding globulin (TBG) in the blood. TBG binds to Synthroid, preventing the drug from working as well to treat your thyroid condition.
What you can do
If you take birth control pills, tell your doctor before starting Synthroid. They’ll likely monitor you more frequently during treatment to ensure that Synthroid is working effectively for you. If needed, they may adjust your dosage of this drug.
Interaction with antacids
Synthroid may interact with antacids, which help treat heartburn.
Examples of antacid medications include:
- calcium carbonate (Tums)
- aluminum hydroxide/magnesium hydroxide (Maalox)
What could happen
Antacids can change the acidity of your stomach. Synthroid needs a certain level of stomach acidity for your body to absorb it. So antacids may decrease the amount of Synthroid absorbed by your body, making Synthroid less effective.
What you can do
To avoid this interaction, it’s important to take antacids at least 4 hours before or after taking Synthroid.
Also, if you take antacids, be sure to tell your doctor before starting Synthroid treatment. They may monitor your thyroid hormone levels more frequently and adjust your Synthroid dosage as needed.
Synthroid may have other interactions. They could occur with supplements, foods, vaccines, or even lab tests. Read on for details. Note that the following information does not include all other possible interactions with Synthroid.
If you have questions about interactions that may affect you, talk with your doctor or pharmacist.
Does Synthroid interact with supplements?
Before you start taking Synthroid, talk with your doctor and pharmacist about any supplements, herbs, and vitamins you take. Sharing this information with them may help you avoid possible interactions.
Synthroid and herbs
There are currently no reports of Synthroid interacting with herbs. But this doesn’t mean that interactions with herbs can’t occur.
For this reason, it’s still important to check with your doctor or pharmacist before taking any of these products during Synthroid treatment.
Synthroid interactions with vitamins and minerals
Some vitamin and mineral supplements may interact with Synthroid, including:
To avoid this interaction, it’s important to take such supplements at least 4 hours before or after taking Synthroid.
Does Synthroid interact with food?
You should take Synthroid on an empty stomach with plain water. Taking Synthroid with any food could decrease the amount of Synthroid your body absorbs. This can make the drug less effective. It’s important to take Synthroid at least 30 minutes to 1 hour before eating food for the first time in a day. It’s best to only drink plain water during this time.
Also, you should avoid certain foods and drinks for 1 hour after taking a dose of Synthroid, including:
- soybeans, soybean flour, or soy-based infant formula
- cottonseed meal
- walnuts
- foods high in dietary fiber
- grapefruit and grapefruit juice
These foods and drinks may decrease Synthroid’s effectiveness. If you regularly consume them, especially within an hour after taking Synthroid, be sure to tell your doctor. They may need to monitor your thyroid hormone levels more frequently and adjust your Synthroid dose.
Does Synthroid interact with vaccines?
There are no known interactions between Synthroid and vaccines. If you need any vaccines while taking Synthroid, talk with your doctor or pharmacist.
Does Synthroid interact with lab tests?
Synthroid may interact with thyroxine-binding globulin tests, which are used to monitor thyroid hormone levels. While you’re taking Synthroid, your doctor may recommend a different test to check thyroid hormone levels.
Does Synthroid interact with cannabis or CBD?Cannabis (commonly called marijuana) and cannabis products, such as cannabidiol (CBD), have been specifically reported to interact with Synthroid. This interaction can cause an increase or decrease in the level of Synthroid in the body.
If cannabis increases the amount of Synthroid in your body, you may experience more side effects from Synthroid. If it decreases the amount of Synthroid in your body, Synthroid may not work as well as it should.
Before you start treatment with Synthroid, tell your doctor and pharmacist if you use cannabis. By sharing this information with them, you may help prevent possible interactions.
Note: Cannabis is illegal at a federal level but is legal in many states to varying degrees.
Certain medical conditions or other health factors may raise the risk of interactions with Synthroid. Before taking Synthroid, discuss your health history with your doctor. They’ll determine whether Synthroid is right for you.
Health conditions or other factors that might interact with Synthroid include:
Heart problems: Synthroid can affect heart function, especially if the dose is too high. If you already have heart problems, taking Synthroid may worsen your condition. Before starting Synthroid treatment, inform your doctor about any existing heart problems you may have. They may recommend starting with a lower dose of Synthroid or monitoring your heart more often throughout treatment.
Untreated adrenal gland problems: Synthroid can affect the amount of hormones that your adrenal glands make. If you have low levels of these hormones, taking Synthroid may worsen your condition. Before taking Synthroid, tell your doctor if you have any untreated adrenal gland issues. They’ll likely treat these problems before prescribing Synthroid.
Diabetes: It’s possible for Synthroid to decrease the effectiveness of diabetes medications or cause higher levels of blood sugar. If you have diabetes, these changes may worsen your condition. Before you start taking Synthroid, tell your doctor if you have diabetes. They may recommend more frequent monitoring of your blood sugar level.
Osteoporosis or other bone conditions: Synthroid may decrease bone density. If you have osteoporosis or another condition affecting your bones, taking Synthroid may worsen your condition. Before taking Synthroid, tell your doctor if you have a condition that weakens bones. If this applies to you, your doctor may recommend more frequent monitoring of your bone health.
Pregnancy: Synthroid is usually considered safe to take during pregnancy. If you’re pregnant or planning to become pregnant, talk with your doctor. They may need to monitor your thyroid hormone levels and adjust your Synthroid dosage more often during pregnancy.
Breastfeeding: Synthroid is usually considered safe to take while breastfeeding. If you’re breastfeeding or planning to breastfeed, talk with your doctor about whether Synthroid is safe for you to take.
Allergic reaction: If you’ve had an allergic reaction to Synthroid or any of its ingredients, your doctor will likely not prescribe Synthroid. This is because taking the drug could cause another allergic reaction. You can ask your doctor about other treatment options that may be better choices for you.
Here are answers to some frequently asked questions about Synthroid and possible interactions.
Is it safe to take Benadryl along with Synthroid?
Yes, it’s safe to take Benadryl (diphenhydramine) with Synthroid. There are no known interactions between the two drugs. Benadryl is an antihistamine that can help treat allergic reactions or seasonal allergies.
Before taking any medications during Synthroid treatment, talk with your doctor or pharmacist to be sure that the medications don’t interact with Synthroid.
If I’m taking Cipro or another antibiotic, will it interact with Synthroid?
There are no known interactions between Synthroid and antibiotics such as Cipro (ciprofloxacin). Antibiotics may be used to treat bacterial infections, such as a urinary tract infection (UTI) or pneumonia.
If you have questions about taking antibiotics during Synthroid treatment, talk with your doctor or pharmacist.
Is it safe to take Synthroid with ashwagandha, collagen, or turmeric supplements?
Yes, it’s likely safe to take ashwagandha, collagen, or turmeric while taking Synthroid. There are no known interactions between these supplements and Synthroid.
However, in general, it’s best to take supplements at a different time of day than your Synthroid dose. This is because the recommendation for many supplements indicates taking them with food. And it’s best to take Synthroid on an empty stomach, at least 30 minutes to 1 hour before eating food for the first time in the day.
Also, Synthroid can interact with other supplements, such as calcium. For more information, see the “Does Synthroid interact with supplements?” section earlier in the article.
Before taking any supplements with Synthroid, be sure to talk with your doctor or pharmacist. They can let you know whether it’s safe to take certain supplements with Synthroid.
Does Synthroid interact with B vitamins, such as biotin or vitamin B12?
No, there are no known interactions between Synthroid and vitamin B12. But biotin (vitamin B7) may interfere with certain thyroid function tests, such as thyroid-stimulating hormone (TSH) and thyroxine (T4) testing. If you take biotin supplements, stop taking them at least 2 days before thyroid tests.
But if you’re taking B vitamins in a multivitamin, be sure to check the product label for other ingredients that may interact with Synthroid. For example, calcium and magnesium can interact with Synthroid. (For more information, see the “Does Synthroid interact with supplements?” section above.)
If you have any questions about which vitamins are safe to take with Synthroid, talk with your doctor or pharmacist.
Are there any interactions between elderberry and Synthroid?
No, there are no known interactions between elderberry and Synthroid. Elderberry is a supplement that is thought to help boost the immune system. It’s likely safe to take this supplement during Synthroid treatment.
If you have any questions about taking elderberry with Synthroid, talk with your doctor or pharmacist.
Taking certain steps can help you avoid interactions with Synthroid. Before starting treatment, talk with your doctor and pharmacist. Things to discuss with them include:
- Whether you drink alcohol or use cannabis.
- Other medications you take, as well as any vitamins, supplements, and herbs. Your doctor or pharmacist can help you fill out a
medication list , including the best time of day to take them. - What to do if you start taking a new drug during your Synthroid treatment.
It’s also important to read Synthroid’s
You may see colored stickers on the label that describe interactions. And the paperwork (sometimes called the patient package insert or medication guide) may have other details about interactions. (If you did not get paperwork with Synthroid, ask your pharmacist to print a copy for you.) If you have trouble understanding this information, your doctor or pharmacist can help.
Taking Synthroid exactly as prescribed can also help prevent interactions.
Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.