Intermittent fasting is gaining some serious momentum. 24% of Americans say they’ve tried it for weight loss and other potential health benefits.[1]

Research shows a link between intermittent fasting and weight loss,[2] heart health,[3] and insulin sensitivity.[4,5] However, as with any new eating pattern, consider the big picture before deciding if it’s right for you.

If you take prescription medications, over-the-counter drugs, or supplements, learn how intermittent fasting could affect the absorption or side effects of these substances.

Worried about keeping track of your fasting hours and supplement / medication timing? If you try the Simple fasting and meal tracking app, you’ll find that you can track your supplements there, too. Take our Simple quiz, and get started today!

Important: 

This article is only for general information and is not meant to replace medical advice already provided. Please consult with your healthcare provider before making any dietary or lifestyle changes or changes to any medications or supplements you take.

How fasting may affect your medication regimen

There are many ways that fasting may affect your medication and supplement routine: 

• It may increase or decrease the effectiveness of your medications or supplements by inhibiting or enhancing absorption.[6] For instance, while some medications are meant to be taken on an empty stomach (which means fasting may not interfere), others are meant to be taken with food.
• Fasting may exacerbate the side effects of certain medications and supplements. For example, nausea or dizziness are common side effects of medications, which fasting may worsen.

You may well be wondering how intermittent fasting affects specific medications and supplements, so let’s break it down. 

We’ll start by diving into supplements — think multivitamins!

Fasting and fat-soluble vitamins 

Vitamins A, D, E, and K are fat-soluble vitamins.[7] This means you absorb them best when you consume them with foods containing fat.[8] 

Depending on where you live, vitamin D is an essential daily supplement over the dark winter months. It’s linked to a range of health benefits, including improved immune system, mood, energy, and respiratory health.[9] Vitamin E is another fat-soluble vitamin that acts as an antioxidant, protecting your cells by fighting free radical damage.[10] 

What this means

If you take vitamins D and E, or a multivitamin that includes D, take them during your eating window, preferably with a meal containing healthy fat like avocado, olive oil, nuts, seeds, fish, or red meat.

Fasting and iron supplements

Iron supplements are often prescribed for people who have low iron. 

While iron is absorbed best on an empty stomach, iron supplements can cause digestive problems like stomach cramps, nausea, diarrhea, and/or constipation. You may need to take iron with a small amount of food to avoid this problem and discuss any stomach problems you may be experiencing with your healthcare provider.

However, iron supplements should not be combined with certain foods. These include: 

• foods or drinks with caffeine
• foods or drinks with calcium, such as dairy products

On the other hand, foods high in vitamin C, such as citrus fruits, do improve iron absorption. So feel free to chase down that iron supplement with a small glass of OJ.[11,12,13]

What this means

Your healthcare provider will likely recommend that you take your iron supplements with food to help alleviate digestive issues. 

They will also likely provide you with a list of foods and drinks to avoid when taking iron. 

Experiment with timing and see what works best for you. 

Fasting and medications 

Diabetes medications

If you live with diabetes or have challenges managing your blood sugar, speak to your doctor before trying intermittent fasting, especially if you take medications like Metformin or insulin. If you take Metformin on an empty stomach, you could get diarrhea or uncomfortable gas. 

If you take diabetes medication during your fast, you could increase your risk of hypoglycemia (aka when the level of glucose in your blood drops below what’s healthy for you). Hypoglycemia can trigger your “fight or flight” response, which can cause fatigue, dizziness, anxiety, and a thumping heart.[14] If your blood sugar remains low, you could experience seizures or other conditions, so talk to your doctor to adjust your medication.

What this means

Consult your healthcare provider before trying intermittent fasting.

(BTW, if you’ve gotten the okay from your healthcare provider to start fasting, tag us in, and we’ll do the heavy lifting in creating a customized plan and meal guide. Take our Simple quiz to get started!)

Thyroid medication

Levothyroxine is a common medication used to treat hypothyroidism, also known as an underactive thyroid. It’s a human-made version of thyroxine, the primary hormone that’s released by your thyroid gland. 

You must carefully consider the timing of this medication. One study indicates that thyroxine absorption is 80% in a fasted state but decreases to 60% in a fed state.[15] Most healthcare providers recommend taking levothyroxine in the morning at least half an hour before breakfast,[16,17] and this seems to be the sweet spot for most people. However, another study of 90 patients found that when levothyroxine was taken on an empty stomach right before bed, it improved thyroid hormone levels.[18] 

What this means

Work with your doctor to find optimal medication timing for your lifestyle. Regardless of timing, the goal is to achieve consistency throughout the week to avoid thyroid level fluctuations. 

Over-the-counter medications

Do you ever reach for an aspirin when you pull a muscle, have a tension headache, or when it’s that time of the month? How about ibuprofen or naproxen? These all fall into the category of over-the-counter NSAIDs (nonsteroidal anti-inflammatory drugs). There’s a consensus that NSAIDs should be taken with food or a glass of milk to minimize side effects like stomach pain, heartburn, nausea, diarrhea, and constipation.[19] 

What this means

It’s usually best to take over-the-counter medications during your eating window. Always check the bottle and follow the instructions.

Foods that may interact with medications 

Some specific foods and nutrients may cause an adverse interaction with particular drugs. 

One of the most common? You might be surprised — it’s grapefruit.

Grapefruit and grapefruit juice

Grapefruits interact with some statins, which are medications used to lower LDL cholesterol. They contain a chemical that can interfere with your body’s ability to break down the statin, leading to an increased level of statin in the blood and possible side effects like rhabdomyolysis — a serious condition where muscles break down and release damaging protein into the blood.[20] 

Grapefruit can also interact with calcium-channel blockers, used for treating high blood pressure and heart disease through the same mechanism of inhibiting breakdown and increasing blood levels of the medication.[21] The increase in blood levels can intensify common side effects like dizziness, constipation, and swelling of lower limbs. 

What this means

Always talk to your doctor or pharmacist about potential drug / nutrient interactions.

Will medications break your fast?

Most medications that come in pill or capsule form will not break a fast.

However, medications that come in chewable, liquid, and/or gummy formats may have small amounts of sugar. For example:

• chewable antacids
• liquid cough syrup
• gummy vitamins

Sugar triggers an insulin response, which will break your fast. Take your gummy multivitamins in your eating window, preferably with food, to increase absorption of those fat-soluble vitamins.