Synthroid (levothyroxine) is a brand-name prescription drug used to treat hypothyroidism and thyroid cancer in certain situations.

As with other medications, Synthroid can interact with certain drugs. It can also interact with some supplements and foods. An interaction occurs when one substance causes another substance to have a different effect than expected.

For details about what interferes with Synthroid, keep reading. For additional information about Synthroid, including details about its uses, see this article.

In some cases, a factor or condition could prevent your doctor from prescribing Synthroid due to the risk of harm. This is known as a contraindication. The contraindications of Synthroid include:

Having untreated adrenal insufficiency

Doctors typically will not prescribe Synthroid if you have adrenal insufficiency that’s not being treated.

With adrenal insufficiency, your body does not produce enough of certain hormones, including cortisol. Synthroid can also reduce the level of certain hormones in your body, which could worsen adrenal insufficiency.

If you have adrenal insufficiency, your doctor may give you treatments for your condition before you start taking Synthroid.

Note: Before you start treatment with Synthroid, it’s important to tell your doctor if this contraindication applies to you. They can determine whether to prescribe Synthroid.

There’s not a known interaction between Synthroid and alcohol. If you drink alcohol, your doctor can advise you on an amount that’s safe to consume while taking the medication.

Before you start treatment with Synthroid, tell your doctor and pharmacist which prescription, over-the-counter, and other medications you take. By sharing this information with them, you may help prevent possible interactions.

If you have questions about drug interactions that may affect you, ask your doctor or pharmacist.

Here’s a chart of drugs that can interact with Synthroid. Doctors may not prescribe the below medications with Synthroid. Keep in mind that this chart does not include all drugs that may interact with Synthroid. Some of these interactions are described in detail just below in “Drug interactions in depth.”

Drug class or drug nameDrug examplesInteraction result with Synthroid
tricyclic antidepressants (TCAs)amitriptyline
• imipramine (Tofranil)
• desipramine (Norpramin)
can increase the effect of TCAs and Synthroid*
beta-blockers• propranolol (Inderal LA, Innopran XL)
• metoprolol (Lopressor, Toprol XL)
• atenolol (Tenormin)
can make beta-blockers less effective than usual
diabetes drugs• canagliflozin (Invokana)
• glimepiride (Amaryl)
metformin (Fortamet, Glumetza, Riomet)
can make diabetes drugs less effective than usual
corticosteroidsprednisone (Rayos)
methylprednisolone (Medrol)
can make Synthroid less effective than usual or increase the risk of side effects from Synthroid*
bile acid sequestrants• colesevelam (Welchol)
• cholestyramine (Prevalite)
• colestipol (Colestid)
can make Synthroid less effective than usual
proton pump inhibitorsomeprazole (Prilosec)
• esomeprazole (Nexium)
• lansoprazole (Prevacid)
can make Synthroid less effective than usual
antacids• sodium bicarbonate (Alka-Seltzer)
• calcium carbonate (Tums)
• aluminum hydroxide/magnesium
hydroxide (Maalox)
can make Synthroid less effective than usual
phosphate binders• lanthanum (Fosrenol)
• sevelamer (Renagel, Renvela)
can make Synthroid less effective than usual
ion exchange resins• sodium polystyrene sulfonate (SPS, Kionex, Kalexate)can make Synthroid less effective than usual
tyrosine kinase inhibitors• imatinib (Gleevec)
• dasatinib (Sprycel)
• nilotinib (Tasigna)
can make Synthroid less effective than usual
certain oral birth control drugs• ethinyl estradiol/norethindrone (Aranelle, Gildagia, others)
• ethinyl estradiol/norgestimate (Estarylla, Sprintec, others)
can make Synthroid less effective than usual
certain seizure drugs• phenytoin (Dilantin)
• carbamazepine (Carbatrol, Epitol, Tegretol, others)
• phenobarbital
can make Synthroid less effective than usual
sucralfate (Carafate)can make Synthroid less effective than usual
orlistat (Alli, Xenical)can make Synthroid less effective than usual
furosemide (Lasix)can make Synthroid less effective than usual
rifampin (Rifadin)can make Synthroid less effective than usual
amiodarone (Nexterone, Pacerone)can make Synthroid less effective than usual, or increase the risk of side effects from Synthroid*
warfarin (Jantoven)can increase the effect of warfarin
digoxin (Lanoxin)can make digoxin less effective than usual
ketamine (Ketalar)can increase the risk of side effects from ketamine

* For more information about the side effects of Synthroid, see this article.

Here’s a closer look at certain drug interactions of Synthroid.

Antacids

Antacids are used to ease symptoms of conditions such as heartburn or indigestion.

Interaction result. Taking Synthroid with an antacid could make Synthroid less effective than usual.

Interaction explained. Synthroid is a synthetic version of a certain thyroid hormone found naturally in the body. When taken together, the antacid may attach to Synthroid and prevent the body from absorbing Synthroid. This lowers the level of Synthroid in your body. Having a low level of Synthroid can keep the drug from working as well as it usually does.

Examples of antacids. Below are examples of antacids that can interact with Synthroid:

  • sodium bicarbonate (Alka-Seltzer)
  • calcium carbonate (Tums)
  • aluminum hydroxide/magnesium hydroxide (Maalox)

Steps you or your doctor may take. Be sure to take Synthroid at least 4 hours before or 4 hours after taking an antacid. This should help prevent the antacid from attaching to Synthroid in your body, allowing Synthroid to be effective for treating your condition.

Certain oral birth control

Oral birth control is medication taken by mouth to help prevent pregnancy. Synthroid may interact with oral birth control that contains a hormone called estrogen.

Interaction result. Taking Synthroid with birth control that contains estrogen could make Synthroid less effective than usual.

Interaction explained. Synthroid is a synthetic version of a certain thyroid hormone found naturally in the body. Taking oral birth control that contains estrogen can increase the level of certain proteins that attach to the thyroid hormone in the body. Increased levels of these proteins can prevent the body from absorbing Synthroid. This keeps the level of the thyroid hormone in your body low. As a result, Synthroid may not work as well as it usually does.

Examples of oral birth control drugs. Here are examples of oral birth control drugs that may interact with Synthroid:

  • ethinyl estradiol/norethindrone (Aranelle, Gildagia, others)
  • ethinyl estradiol/norgestimate (Estarylla, Sprintec, others)

Steps you or your doctor may take. Before taking Synthroid, tell your doctor if you take oral birth control. They’ll help determine whether it contains estrogen. If it does, your doctor may prescribe a higher dosage of Synthroid than usual. For details about Synthroid’s dosage, see this article.

Orlistat

Orlistat (Alli, Xenical) is a medication used for weight loss.

Interaction result. Taking Synthroid with orlistat could make Synthroid less effective than usual.

Interaction explained. Synthroid is a synthetic version of a certain thyroid hormone found naturally in the body. When taken together, orlistat may attach to Synthroid and prevent the body from absorbing Synthroid. This lowers the level of Synthroid in your body, which prevents Synthroid from working as well as it usually does.

Steps you or your doctor may take. Before starting Synthroid treatment, be sure to tell your doctor if you’re taking orlistat. They can advise you on whether it’s safe to take these medications together.

If your doctor prescribes Synthroid with orlistat, you’ll take orlistat at least 4 hours before or 4 hours after taking Synthroid. This should help prevent orlistat from attaching to Synthroid in the body, allowing Synthroid to be effective for treating your condition.

Synthroid may have other interactions, such as with supplements, foods, vaccines, or even lab tests. You’ll find details below.

Synthroid interactions with supplements

Certain supplements may interact with Synthroid. These include iron and calcium supplements. If either supplement is taken with Synthroid, it could decrease the level of the drug in your body. This may make Synthroid less effective than usual.

Due to this risk, you should not take iron or calcium supplements at least 4 hours before or 4 hours after taking Synthroid. This should help prevent iron or calcium from affecting the level of Synthroid in your body. As a result, Synthroid can work to be effective for treating your condition.

Before you start treatment with Synthroid, tell your doctor and pharmacist which supplements, herbs, and vitamins you take. By sharing this information with them, you may help prevent possible interactions.

If you have questions about drug interactions that may affect you, ask your doctor or pharmacist.

Synthroid and herbs

There have not been specific reports of herbs interacting with Synthroid. However, it’s still important to check with your doctor or pharmacist before taking any of these products during Synthroid treatment.

Synthroid and vitamins

Vitamins have not been reported to interact with Synthroid. However, multivitamins may contain iron and calcium, which are known to interact with Synthroid. For details, see “Synthroid and supplement interactions” above.

Before taking any vitamins during Synthroid treatment, talk with your doctor or pharmacist.

Synthroid interactions with food

Certain foods and drinks could decrease the level of Synthroid in your body. This could make Synthroid less effective than usual. For this reason, it’s best to take Synthroid on an empty stomach. You’ll take the drug at least 30 minutes to 1 hour before your first meal of the day.

Examples of foods and drinks that could make Synthroid less effective include:

  • grapefruit and grapefruit juice
  • soy-based flour, such as that used in infant formula
  • cottonseed flour
  • walnuts
  • high-fiber foods, such as sweet potatoes, black beans, and avocado
  • coffee
  • foods or drinks that contain calcium,* such as milk and other dairy products

If you have questions about foods or drinks to avoid with Synthroid, talk with your doctor or pharmacist.

* To learn more, see “Synthroid interactions with supplements” above.

Synthroid and vaccines

Vaccines and Synthroid have not been reported to interact with each other. For more information about getting certain vaccines while you take Synthroid, talk with your doctor or pharmacist.

Synthroid interactions with lab tests

No lab tests have been reported to interact with Synthroid. Your doctor or pharmacist can tell you more about having certain lab tests during Synthroid treatment.

Synthroid interaction with cannabis or CBD

Cannabis (marijuana) and cannabis products, such as cannabidiol (CBD), have been specifically reported to interact with Synthroid.

The effects of cannabis on Synthroid can vary depending on the exact cannabis product used. Certain cannabis products may decrease the level of Synthroid in the body, which could cause Synthroid to be less effective than usual.

Other cannabis products may raise the level of Synthroid in the body, which could increase your risk of side effects from Synthroid. (To learn about possible side effects of Synthroid, see this article.)

Before you start treatment with Synthroid, tell your doctor and pharmacist if you take 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 and other factors may increase the risk of interactions with Synthroid. Before you take Synthroid, be sure to talk with your doctor about your health history. Synthroid may not be the right treatment option for you if you have certain medical conditions or other factors affecting your health.

Health conditions or factors that might interact with Synthroid include:

Diabetes. Before starting Synthroid treatment, tell your doctor if you have diabetes. Synthroid can make it harder to manage your blood sugar level. For this reason, your doctor may have you check your blood sugar level more often than usual while you’re taking Synthroid. If you’re taking diabetes medication, they may adjust the dosage to help manage your blood sugar level.

Heart problems. Before taking Synthroid, tell your doctor if you have heart problems, such as coronary artery disease. Synthroid may cause heart problems as a side effect, which could worsen any heart problems you already have. Examples of these side effects include arrhythmia and heart attack. To decrease your risk of heart problems with Synthroid, your doctor may prescribe a lower dosage than usual. For details about Synthroid’s dosage, see this article.

Being age 65 years or older. Synthroid may cause heart problems as a side effect. Examples include arrhythmia and heart attack. Your risk of these side effects may be increased if you’re age 65 years or older. To reduce this risk, doctors may prescribe a lower dosage of Synthroid for people in this age range. For details about Synthroid’s dosage, see this article.

Untreated adrenal insufficiency. If you have adrenal insufficiency that’s not being treated, doctors typically will not prescribe Synthroid. For details, see the “When to avoid Synthroid” section above.

Pregnancy. Doctors typically consider Synthroid safe to take during pregnancy. For details about taking Synthroid during pregnancy, see this article.

Breastfeeding. Synthroid passes into breast milk. However, it’s not known if the drug causes side effects in a breastfed child. For details about taking Synthroid while breastfeeding, see this article.

Allergic reaction. If you’ve had an allergic reaction to Synthroid or any of its ingredients, your doctor will likely not prescribe Synthroid. Taking the drug could cause another allergic reaction. You can ask them about other treatments that may be better choices for you.

Having gone through menopause. Synthroid may cause low bone density (weakened bones) as a side effect. The risk of this side effect may be highest in females* who have gone through menopause.

If you’ve gone through menopause, tell your doctor before starting Synthroid treatment. They may give you a bone density test from time to time. If you’re found to have low bone density, your doctor may adjust your Synthroid dosage. For details about Synthroid’s dosage, see this article.

* Sex and gender exist on spectrums. Use of the term “female” in this article refers to sex assigned at birth.

Here are some frequently asked questions about Synthroid and possible interactions.

Does Synthroid interact with supplements that may help with arthritis, such as ashwagandha, collagen, or turmeric?

Synthroid is not known to interact with these supplements.

Ashwagandha is used to ease symptoms of various health conditions, including arthritis, anxiety, and stress. Collagen supplements are used to improve skin and joint health. Turmeric may be used to reduce inflammation, ease pain, and improve digestion.

To learn more about Synthroid and these supplements, talk with your doctor or pharmacist. They can advise you on whether it’s safe to take any of them during Synthroid treatment.

Do B vitamins such as biotin or B12 interact with Synthroid?

Synthroid is not known to interact with B vitamins such as biotin and vitamin B12.

Biotin and vitamin B12 are used for various health conditions, including improving the health of hair, skin, and nails.

To learn more about taking Synthroid with biotin or vitamin B12, talk with your doctor or pharmacist.

Is it safe to take Synthroid with Benadryl?

Yes, it’s believed that Synthroid is safe to take with Benadryl (diphenhydramine). Synthroid and Benadryl are not known to interact with each other.

Benadryl is a type of medication called an antihistamine. It’s used to treat conditions such as seasonal allergies.

If you’re interested in an allergy treatment while taking Synthroid, talk with your doctor or pharmacist. They can help answer other questions you may have about Synthroid and Benadryl.

Can I take Synthroid with Cipro?

It’s thought that Synthroid is safe to take with Cipro (ciprofloxacin). There’s no known interaction between these two drugs.

Cipro is an antibiotic and is used to treat bacterial infections.

If you’d like to learn more about Synthroid and Cipro, talk with your doctor or pharmacist.

You can take certain steps to help prevent interactions with Synthroid. Your doctor and pharmacist are key resources, so reach out to them before starting treatment. For example, you should plan the following:

  • Let them know if you take cannabis.
  • Tell them about any other medications you take, as well as any supplements, herbs, and vitamins.
  • Create a medication list, which your doctor and pharmacist can help you fill out.

It’s also important to read the label of Synthroid and other paperwork that comes with the drug. The label may have colored stickers that mention an interaction. And the paperwork, sometimes called the prescribing information, may contain details about interactions. If this information is difficult to understand, ask your doctor or pharmacist to help explain it.

You can also help prevent interactions with Synthroid by taking it exactly as your doctor prescribes.

Besides learning about interactions, you may want to find out more about Synthroid. These resources might help:

Disclaimer: Medical News Today 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.