What class of drug is Susten? A Comprehensive Guide to Progesterone Medication

October 7, 2025 •

4 min read

Low progesterone levels can lead to menstrual irregularities and difficulty conceiving [1.8.2, 1.8.5]. So, what class of drug is Susten? It belongs to a class of medications called progestins, which are female hormones containing natural micronized progesterone [1.2.3, 1.4.5].

Quick Summary

Susten is classified as a progestin, a female hormone. Its active ingredient is natural micronized progesterone, used for hormone replacement therapy, treating female infertility, and supporting pregnancy [1.2.1, 1.2.2, 1.3.2].

Key Points

Drug Class: Susten belongs to the class of medicines known as progestins, which are female hormones [1.2.3].
Active Ingredient: Its active component is natural micronized progesterone, which is structurally identical to the hormone produced by the human body [1.3.2, 1.4.5].
Primary Uses: It is used for hormone replacement therapy (HRT), treating female infertility, supporting early pregnancy, and regulating menstrual disorders like amenorrhea [1.4.1, 1.4.2, 1.4.3].
Mechanism of Action: Susten works by preparing the uterine lining for implantation, maintaining pregnancy, and balancing the effects of estrogen [1.5.2, 1.5.6].
Enhanced Absorption: The progesterone in Susten is 'micronized,' meaning its particles are reduced in size to improve dissolution and absorption in the body [1.2.2].
Administration Forms: It is available in various forms, including oral capsules, sustained-release tablets, injections, and vaginal gels [1.2.5].
Important Precautions: Susten is contraindicated in patients with a history of breast cancer, undiagnosed vaginal bleeding, severe liver disease, or blood clotting disorders [1.6.1, 1.6.2].

Understanding Susten and Its Classification

Susten is a brand name for a medication whose active ingredient is progesterone [1.3.2]. It falls under the drug class known as progestins, which are natural or synthetic forms of this crucial female hormone [1.2.3, 1.2.6]. Specifically, Susten contains natural micronized progesterone, a form derived from plants and processed to have a smaller particle size, which enhances its absorption and bioavailability in the body [1.2.2, 1.4.5].

Progesterone itself is a naturally occurring steroid hormone produced mainly by the ovaries' corpus luteum after ovulation, and by the placenta during pregnancy [1.8.4]. It plays a fundamental role in regulating the menstrual cycle and preparing the body for and maintaining a pregnancy [1.8.3, 1.8.4]. Susten is prescribed to supplement the body's natural progesterone levels when they are insufficient [1.4.1].

Mechanism of Action: How Susten Works

The primary function of Susten is to mimic the effects of natural progesterone. Its mechanism of action involves several key processes in the female reproductive system:

Endometrial Preparation It prepares the lining of the uterus (endometrium) for a fertilized egg to implant. It does this by transforming the endometrium from a proliferative (growing) state to a secretory state, making it receptive to an embryo [1.4.3, 1.5.2].
Pregnancy Maintenance Once pregnancy occurs, Susten helps maintain the uterine lining, provides a favorable hormonal environment, and prevents uterine contractions that could lead to miscarriage or premature labor [1.4.1, 1.5.3, 1.5.6].
Menstrual Cycle Regulation By supplementing progesterone, Susten helps regulate the menstrual cycle, particularly in cases of amenorrhea (absence of periods) where a lack of progesterone is the cause [1.4.1, 1.4.5].
Balancing Estrogen In hormone replacement therapy (HRT), it is often used with estrogen to counteract estrogen's proliferative effect on the uterine lining, which can reduce the risk of endometrial hyperplasia (abnormal thickening) and uterine cancer [1.4.3, 1.4.5].

Common Uses and Medical Indications

Due to its role in the reproductive system, Susten is prescribed for a variety of conditions related to hormonal imbalances:

Hormone Replacement Therapy (HRT): It is used for postmenopausal women, often in combination with estrogen, to relieve symptoms like hot flashes, night sweats, and mood swings [1.4.3, 1.4.5].
Female Infertility: It provides luteal phase support during assisted reproductive technologies (ART) like IVF to increase the chances of successful implantation and pregnancy [1.4.2, 1.4.3].
Pregnancy Support: It is used to prevent recurrent miscarriages due to progesterone deficiency and to prevent preterm labor in high-risk pregnancies [1.4.1, 1.4.5].
Menstrual Disorders: The drug is used to treat secondary amenorrhea (absence of periods) and dysfunctional or abnormal uterine bleeding caused by hormonal imbalance [1.4.1, 1.4.3].

Available Forms and Administration

Susten is available in several forms to suit different clinical needs, including:

Oral capsules (often soft gelatin) [1.2.5]
Sustained-release (SR) tablets [1.4.1]
Vaginal gels or pessaries [1.2.5]
Injectable solutions [1.2.5, 1.4.4]

Oral capsules are typically taken with food to enhance absorption, and sometimes at bedtime to minimize side effects like dizziness or drowsiness [1.7.3, 1.4.1]. The specific dosage, form, and duration of treatment are determined by a healthcare provider based on the individual's medical condition [1.7.4].

Comparison with Other Progesterone Medications

Susten is a form of natural micronized progesterone. It is often compared to synthetic progestins and other forms of progesterone administration.


Feature	Susten (Natural Micronized Progesterone)	Synthetic Progestins	Other Progesterone Forms
Source	Plant-derived, chemically identical to human progesterone [1.2.2].	Artificially created in a lab.	Can be natural or synthetic.
Bioavailability	Micronization process enhances dissolution and absorption [1.4.5].	Varies by specific drug.	Sustained-release (SR) versions offer gradual release [1.9.4]. Injections and gels bypass first-pass metabolism.
Side Effects	Generally considered safer with fewer side effects compared to some synthetic versions [1.2.2]. Common effects include dizziness, bloating, and headache [1.6.6].	Can have different side effect profiles; some may have androgenic effects.	Injections can cause site pain [1.5.2]. Vaginal gels can cause local irritation.
Primary Use	HRT, fertility support, pregnancy maintenance [1.4.5].	Often used in contraceptives and HRT.	Similar to Susten; depends on formulation (e.g., injections for ART) [1.9.3].

Natural micronized progesterone in a sustained-release (NMP-SR) formulation has shown benefits over conventional oral forms due to better bioavailability and tolerability with once-daily dosing [1.9.4].

Potential Side Effects and Precautions

While generally well-tolerated, Susten can cause side effects [1.6.2].

Common Side Effects:

Headache [1.6.4]
Dizziness and drowsiness [1.6.1]
Abdominal bloating and cramps [1.4.5, 1.6.6]
Breast tenderness or pain [1.6.4]
Mood swings or depression [1.6.2, 1.6.4]
Irregular vaginal bleeding or spotting [1.4.5]

Serious Side Effects & Contraindications: Susten should not be used by individuals with certain conditions, including:

Known allergy to progesterone or its ingredients [1.6.2]
A history of breast cancer or other hormone-dependent cancers [1.6.1, 1.6.5]
Undiagnosed vaginal bleeding [1.6.2]
Severe liver disease [1.6.2]
A history of blood clots (thromboembolism) or arterial disease [1.6.1, 1.6.2]

Patients should inform their doctor about their full medical history, including conditions like diabetes, epilepsy, migraine, or depression before starting treatment [1.6.1, 1.6.6].

Conclusion

In summary, Susten is classified as a progestin, containing natural micronized progesterone. It is a vital medication in modern gynecology and reproductive medicine, used to correct progesterone deficiencies that cause infertility, menstrual disorders, and menopausal symptoms. By mimicking the body's natural hormone, it helps prepare the uterus for pregnancy, maintain it, regulate cycles, and balance the effects of estrogen in hormone replacement therapy. While effective, it must be used under the guidance of a healthcare professional due to potential side effects and contraindications.

For more information on progesterone, you can visit the Endocrine Society's page on Reproductive Hormones [1.8.5].

Frequently Asked Questions

Susten is used to treat conditions caused by a lack of progesterone, such as female infertility, irregular menstrual periods (amenorrhea), abnormal uterine bleeding, and for hormone replacement therapy in menopausal women. It is also used to support pregnancy and prevent miscarriage [1.2.1, 1.4.1, 1.4.5].

Yes, its active ingredient, progesterone, is a steroid hormone that belongs to a class of hormones called progestogens [1.8.5].

Yes, taking Susten may cause weight gain, which can be due to water retention and may not be a serious sign. If weight gain is concerning, you should consult your doctor [1.2.2, 1.4.5].

Susten capsules should be swallowed whole with water and not chewed or crushed. It is often recommended to be taken with food to improve absorption and at bedtime to reduce potential side effects like dizziness [1.7.3, 1.4.1].

Common side effects include headache, breast pain, bloating, nausea, dizziness, drowsiness, and mood swings. These are often more common in the first few weeks of treatment [1.4.5, 1.6.6].

Susten is often prescribed during pregnancy to support the uterine lining and prevent miscarriage, especially in cases of progesterone deficiency or for women with a history of recurrent miscarriages. It should only be used under a doctor's guidance [1.4.1, 1.4.2].

Micronized progesterone means the natural progesterone particles have been reduced in size. This process enhances the drug's dissolution and absorption in the body, which can improve its effectiveness and reduce side effects [1.2.2, 1.4.5].