The menstrual cycle is a finely tuned process, typically lasting an average of 28 days, though a normal range extends from 21 to 35 days. A period is generally considered late if it does not arrive within this expected timeframe or if it is more than five to seven days past the usual start date. When a period is delayed or missed, the most common concern is pregnancy, but many factors can disrupt this biological rhythm. Certain medications are known to interfere with the complex hormonal signals that govern the menstrual cycle, leading to a delayed or absent period. Understanding how different drug classes interact with the body’s reproductive system can help clarify why this side effect occurs.
Direct Hormonal Interference
Medications designed to alter reproductive timing often work by directly manipulating the Hypothalamic-Pituitary-Ovarian (HPO) axis, the communication pathway between the brain and the ovaries. The HPO axis relies on the precise, pulsatile release of Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus, which then triggers the pituitary gland to release Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). Hormonal contraceptives, such as oral pills, implants, and injections, introduce synthetic estrogen and progestin.
These exogenous hormones suppress the release of GnRH, blocking the mid-cycle surge of LH and FSH necessary for ovulation. Without ovulation, the normal cycle progression is interrupted, resulting in a delayed or absent withdrawal bleed, which is the period-like bleeding experienced during the placebo week of many contraceptives. Contraceptives containing only progestin, like the injection or implant, can cause the uterine lining to thin significantly, often leading to the complete absence of a period (amenorrhea). Emergency contraception pills contain a high dose of hormones that temporarily override the body’s natural cycle timing, causing a short-term delay or change in the next expected period.
Indirect Influence on Cycle Regulation
Some medications delay periods not by introducing sex hormones, but by interfering with brain chemistry and the body’s stress response system. This mechanism targets neurotransmitter pathways that indirectly influence the HPO axis. For instance, certain psychiatric medications, particularly antipsychotics and some antidepressants, can cause an increase in the hormone prolactin.
Dopamine typically acts as an inhibitor of prolactin secretion. Many antipsychotic medications block dopamine receptors, essentially removing the “brake” on prolactin production. Elevated prolactin levels, a condition called hyperprolactinemia, suppress the release of GnRH from the hypothalamus. This suppression leads to a decrease in LH and FSH, which prevents the ovarian follicle from maturing and releasing an egg, ultimately delaying or stopping the period.
High-dose corticosteroids, often used to treat inflammation or autoimmune conditions, can mimic a severe stress response. This stress signal prompts the hypothalamus to prioritize survival over reproduction, leading to the temporary shutdown of the reproductive axis. These drugs can induce irregular or missed periods through an indirect, stress-based mechanism.
Other Medical Treatments and Cycle Changes
A separate category of medical treatments can disrupt the menstrual cycle as a systemic side effect, often independent of direct hormonal or neurotransmitter manipulation. Chemotherapy drugs, for example, are designed to target and destroy rapidly dividing cells, which includes the cells within the ovaries that contain the eggs. Damage to ovarian tissue can lead to a sudden decline in estrogen production, causing periods to become irregular, stop completely, or trigger premature menopause.
Other systemic treatments can alter the metabolic and hormonal environments that regulate the cycle. Thyroid medications, particularly during the initial phase of dose adjustment, may cause cycle irregularities because thyroid hormones are fundamentally important for reproductive function. Anti-epileptic drugs (AEDs) can also interfere with the liver’s ability to metabolize sex hormones, changing their circulating levels and potentially leading to delayed or absent periods. When cycle disruption occurs with these treatments, it is an unintended consequence of treating a major illness, and any changes warrant immediate consultation with a healthcare provider.
When Medication Is Not the Cause
A delayed period is often caused by factors unrelated to medication. The menstrual cycle is highly sensitive to changes in the body’s internal and external environment. Psychological or physical stress can signal the body to conserve energy, leading the hypothalamus to temporarily reduce the production of GnRH and delay ovulation.
Significant fluctuations in body weight, whether rapid gain or loss, can also cause hormonal imbalances that disrupt the cycle. Low body weight, often seen in cases of disordered eating or excessive exercise, can reduce the body fat necessary for hormone production, halting menstruation. Conversely, a high body weight can lead to an overabundance of estrogen, which causes cycle irregularity. Common conditions like Polycystic Ovary Syndrome (PCOS) or thyroid disorders are frequent non-drug causes of missed periods. If a period is missed three times in a row, or if the delay is accompanied by other concerning symptoms, seeking advice from a healthcare professional is the appropriate next step.