Kelnor 1/35 28 Day Drug Interactions: What to Avoid and What to Tell Your Doctor

Kelnor 1/35 28 Day can interact with a range of prescription medications, over-the-counter drugs, herbal supplements, and other substances. Some interactions reduce the effectiveness of Kelnor 1/35 — potentially leading to an unintended pregnancy. Others can cause serious side effects. This guide covers the most important interactions you need to know about in 2026.

Always tell your prescriber and pharmacist all medications you take — including over-the-counter drugs, vitamins, and herbal supplements — before starting Kelnor 1/35.

Major Interactions (Can Significantly Reduce Effectiveness or Cause Serious Harm)

Rifampin (Rifampicin)

Rifampin is an antibiotic used to treat tuberculosis and some other serious bacterial infections. It significantly reduces the blood levels of both ethinyl estradiol and progestins by strongly inducing liver enzymes that metabolize the hormones faster. Women taking rifampin should use a backup contraceptive method during treatment and for at least one month after completing the course.

Hepatitis C Combination Regimens (Ombitasvir/Paritaprevir/Ritonavir ± Dasabuvir)

Kelnor 1/35 carries a specific warning about this combination. Co-administration with ombitasvir/paritaprevir/ritonavir (with or without dasabuvir, used to treat Hepatitis C) can cause dramatically elevated liver enzymes (ALT), in some cases exceeding 20 times the upper limit of normal. Kelnor 1/35 must be discontinued before starting this Hepatitis C treatment and can be resumed approximately 2 weeks after completing the regimen.

Moderate Interactions (May Reduce Effectiveness — Use Backup Contraception)

Antiepileptic Drugs (AEDs)

Several anti-seizure medications are enzyme inducers that can reduce the effectiveness of hormonal contraceptives:

Phenytoin (Dilantin) — induces liver enzymes, lowers contraceptive hormone levels

Carbamazepine (Tegretol) — reduces effectiveness of combined oral contraceptives

Phenobarbital — barbiturate that accelerates hormone metabolism

Oxcarbazepine, primidone, topiramate (at doses >200 mg/day) — also reduce contraceptive efficacy

Women taking enzyme-inducing AEDs should use a non-hormonal backup method or discuss alternative contraceptive options with their neurologist and OB/GYN.

Certain HIV Medications (Antiretrovirals)

Some antiretroviral drugs used to treat HIV can interact with oral contraceptives. Atazanavir (without ritonavir) increases ethinyl estradiol levels. Atazanavir with ritonavir decreases ethinyl estradiol levels. Ritonavir-boosted regimens generally reduce oral contraceptive efficacy. Women living with HIV should discuss contraceptive options with their infectious disease provider.

Griseofulvin (Antifungal)

Griseofulvin (used for fungal infections of skin, hair, and nails) may reduce the effectiveness of oral contraceptives. Use backup contraception during and for one month after griseofulvin treatment.

Aromatase Inhibitors

Aromatase inhibitors like anastrozole (Arimidex) and exemestane (Aromasin) are used in breast cancer treatment. They work by blocking estrogen production, directly opposing the estrogen in Kelnor 1/35. These drugs should not generally be taken together — discuss with your oncologist.

Minor Interactions (Generally Less Clinically Significant, but Worth Knowing)

Certain Antibiotics

The prescribing information for Kelnor 1/35 notes a possible (though less marked) association between reduced efficacy and the use of ampicillin and tetracyclines. Current evidence suggests most broad-spectrum antibiotics do not meaningfully reduce oral contraceptive efficacy in most women. However, some providers recommend backup contraception as a precaution during antibiotic treatment.

St. John's Wort (Hypericum perforatum)

St. John's Wort is an herbal supplement commonly used for mild depression. It is a known enzyme inducer and can reduce blood levels of both estrogen and progestin components in oral contraceptives, potentially reducing effectiveness. Avoid concurrent use or discuss with your provider.

What to Tell Your Doctor Before Starting Kelnor 1/35

Before starting Kelnor 1/35, give your prescriber a complete medication list including:

All prescription medications (including antibiotics, antiepileptics, antiretrovirals, antifungals)

Over-the-counter medications (including NSAIDs, antacids, antihistamines)

Herbal supplements, especially St. John's Wort, echinacea, or saw palmetto

Vitamins and dietary supplements in high doses

Also related: Kelnor 1/35 side effects: What to expect and when to call your doctor.