timing
10 interactions related to timing
methotrexate + folate
Methotrexate works by blocking the enzyme that recycles folate into its active form, which depletes folate in normal tissues and drives common side effects such as nausea, mouth sores, and elevated liver enzymes. Folic acid supplementation reduces these side effects without compromising efficacy at the doses used for autoimmune disease, but it should not be taken on the same day as methotrexate, and it should never be added on your own when methotrexate is used for cancer.
levothyroxine + soy
Soy protein and isoflavones can bind to levothyroxine in the gut and reduce how much of the dose is absorbed, which can raise TSH and, in some people, increase the dose needed to stay in range. The effect is most relevant with large, variable soy intake taken close to the dose, and is best documented in infants fed soy formula.
liothyronine + calcium
Calcium salts (carbonate, citrate, acetate) can bind thyroid hormone in the gut and reduce absorption of liothyronine (T3), much as they do with levothyroxine. Taking them together can blunt the effect of the dose and lead to less reliable thyroid replacement.
liothyronine + iron
Iron salts (ferrous sulfate, fumarate, gluconate, bisglycinate) can bind thyroid hormone in the gut and reduce absorption of liothyronine, similar to the well-documented interaction with levothyroxine. Taking them at the same time can blunt the T3 effect and let TSH drift upward.
yogurt + antibiotics
The calcium in yogurt can bind to certain antibiotics — specifically the tetracyclines and fluoroquinolones — in the gut and reduce how much of the drug is absorbed. This is the same chelation interaction seen with milk. Penicillins and macrolides are not meaningfully affected. The fix is timing: take these antibiotics with water and keep yogurt and other calcium-rich foods a couple of hours apart from the dose.
kefir + antibiotics
Antibiotics can kill the live bacteria and yeast in kefir if both are taken at the same time, and dairy calcium can bind certain antibiotics (tetracyclines, fluoroquinolones) and reduce their absorption. Spacing kefir a couple of hours away from each dose addresses both concerns. Note that the broader claim that kefir prevents antibiotic-associated diarrhea is not well supported: the one randomized trial of kefir itself found no benefit.
levothyroxine + fiber
Dietary and supplemental fiber can bind levothyroxine in the gut and modestly reduce how much of each dose is absorbed. When fiber intake is high or variable around the time of dosing, this can nudge TSH upward and make a stable dose harder to settle on. The effect is real but generally modest, and it is managed mainly by timing and consistency rather than by avoiding fiber.
levothyroxine + coffee
Coffee can reduce how much levothyroxine you absorb when the two are taken at the same time. Chlorogenic acids and other compounds in coffee appear to bind the hormone in the gut, and coffee can also speed gastric transit, leaving less time for the tablet to dissolve. The effect largely disappears when the dose and the coffee are separated by enough time.
phenytoin + calcium
Calcium-containing supplements and antacids can bind phenytoin in the gut and lower how much of the drug is absorbed when the two are taken together, which can reduce phenytoin's blood level. Separately, long-term phenytoin use can reduce calcium absorption by speeding up the breakdown of vitamin D, which is relevant to bone health over time.
coffee + iron
Coffee contains chlorogenic acid and other polyphenols with galloyl groups that bind non-heme iron in the gut, forming poorly soluble complexes the intestine cannot absorb. Drinking coffee with or shortly after an iron-rich meal or supplement meaningfully reduces how much non-heme iron you take up.