The “1/35” on your birth control pill refers to the doses of the two hormones inside each tablet: 1 mg of a progestin (norethindrone or a similar synthetic progesterone) and 0.035 mg (35 micrograms) of ethinyl estradiol, a synthetic estrogen. It’s a shorthand printed on the packaging so pharmacists, doctors, and patients can quickly identify the exact hormone strength.
Breaking Down the Numbers
The first number, 1, stands for 1 milligram of the progestin component. In most 1/35 formulations that progestin is norethindrone, though one version (Kelnor 1/35) uses a closely related compound called ethynodiol diacetate. Progestin is the hormone primarily responsible for preventing ovulation and thickening cervical mucus so sperm can’t easily reach an egg.
The second number, 35, stands for 35 micrograms (0.035 mg) of ethinyl estradiol, the estrogen component. Estrogen helps stabilize the uterine lining, which reduces breakthrough bleeding and keeps your cycle regular. Together the two hormones work as a combination oral contraceptive, commonly called “the pill.”
Where 1/35 Falls on the Dose Spectrum
Birth control pills are classified by how much estrogen they contain. Anything under 50 micrograms of ethinyl estradiol is considered low-dose, and 35 micrograms sits right at the upper boundary of that low-dose range. Pills with 50 micrograms (the older “1/50” formulations) are generally no longer recommended for routine contraception and are mostly reserved for treating acute uterine bleeding.
On the lower end, pills with 20 micrograms of estrogen are also widely prescribed. Research shows similar contraceptive effectiveness between 20-microgram and 35-microgram pills, but some data suggest a slightly lower risk of blood clots with 20-microgram formulations. The tradeoff is that lower-estrogen pills tend to cause more spotting and breakthrough bleeding, especially in the first few months. A 35-microgram pill like a 1/35 formulation generally does a better job of keeping bleeding predictable, which is one reason doctors still prescribe it frequently.
Common Brand Names
You’ll find the 1/35 formulation sold under many names. Some of the most recognized include Ortho-Novum 1/35, Norinyl 1+35, Nortrel 1/35, Brevicon, Modicon, Balziva, and Ovcon 35. Generic versions are widely available and are labeled as norethindrone and ethinyl estradiol tablets. Pirmella 1/35 is another common generic. Despite the different packaging, all of these contain the same active hormone doses: 1 mg of progestin and 35 mcg of estrogen per active pill.
How the Pack Is Set Up
Most 1/35 pills come in either a 21-day or 28-day pack. In a 21-day pack, every pill is an active hormone tablet. You take one pill daily for three weeks, then take no pills for seven days while you have your withdrawal bleed. A 28-day pack includes the same 21 active pills plus 7 inactive (placebo) tablets for the final week. The placebo pills contain no hormones and exist purely to keep you in the habit of taking a pill every day so you don’t lose track of your schedule.
Benefits Beyond Contraception
Combination pills in this dose range are associated with several non-contraceptive benefits. They tend to make periods lighter and more regular, which can reduce the risk of iron-deficiency anemia in people with heavy menstrual bleeding. They also decrease the frequency of painful cramps (dysmenorrhea). Some providers prescribe 1/35 formulations to help manage acne, though this effect varies from person to person and depends on the specific progestin used.
Blood Clot Risk at 35 Micrograms
All combination birth control pills raise the risk of venous thromboembolism (blood clots, typically in the legs or lungs) compared to not taking hormonal contraception. This risk is dose-dependent, meaning higher estrogen levels carry higher risk. Pills containing 30 to 35 micrograms of ethinyl estradiol carry a moderately higher clot risk than 20-microgram pills, and pills at 50 micrograms carry higher risk still.
In absolute terms, the risk remains small: roughly 1 to 3 blood clot events per 10,000 women per year of use. For comparison, pregnancy itself carries a substantially higher clot risk than any birth control pill. The type of progestin also matters. Newer progestins like desogestrel and drospirenone are associated with a 50% to 80% higher clot risk compared to older progestins like levonorgestrel. Norethindrone, the progestin in most 1/35 pills, is an older-generation progestin and falls on the lower end of that risk spectrum.
How 1/35 Compares to 1/50
If you’ve seen a 1/50 formulation and wondered how it differs, the progestin dose is the same (1 mg) but the estrogen is higher at 50 micrograms. A large multicenter trial comparing the two found no significant difference in overall effectiveness or discontinuation rates over 12 months. The main differences were practical: women on the 1/35 pill experienced more spotting between periods, while women on the 1/50 pill reported more breast discomfort. Because the higher estrogen dose also raises clot risk without improving effectiveness, current guidelines recommend prescribing pills with 35 micrograms or less for routine contraception.