Methotrexate (MTX) is a medication used to treat a variety of conditions, ranging from certain cancers to autoimmune disorders like rheumatoid arthritis (RA) and psoriasis. As a disease-modifying antirheumatic drug, or DMARD, it works by slowing the immune system activity that causes inflammation and joint or tissue damage. Adherence to the prescribed dosing schedule is paramount for both effectiveness and safety. Missing a dose, or taking it incorrectly, can compromise the therapeutic effect or lead to serious side effects.
Understanding Methotrexate Dosing Schedules
Methotrexate dosing varies drastically depending on the condition being treated and the dosage strength. For autoimmune conditions such as RA, psoriatic arthritis, and severe psoriasis, MTX is almost always prescribed as a single, low dose taken once every seven days. This weekly schedule is designed to allow the body enough time to process the drug while maintaining a steady anti-inflammatory effect.
The once-weekly regimen is a safety mechanism that prevents the medication from accumulating in the body to toxic levels. In contrast, Methotrexate is sometimes used in much higher doses, and occasionally on a daily schedule, to treat certain types of cancer. This distinction is a common point of confusion that carries a significant risk. Taking a weekly dose on a daily basis, even at the low-dose strength used for arthritis or psoriasis, can lead to a fatal overdose.
Immediate Steps After Realizing a Missed Dose
If you realize you have missed your scheduled weekly dose of Methotrexate, the immediate action depends entirely on how much time has passed. The most common advice for the weekly regimen involves a narrow window of opportunity to safely catch up. If you remember within 48 hours of your usual scheduled time, you should take the dose immediately.
Taking the dose within this short window helps maintain the necessary concentration of the drug in your system without compromising the safety margin before the next scheduled dose. After taking the late dose, reset your clock and take your next dose on the originally scheduled day of the following week. This allows the full seven days between doses to pass.
If more than 48 hours have passed since your scheduled dose, you must skip the missed dose entirely. Taking the dose when the gap until the next scheduled week is too short can cause the drug to accumulate, increasing the risk of toxicity. Instead, wait for your next regularly scheduled day to take your dose, ensuring a full week has elapsed since the missed dose was due. Never take a double dose to make up for the one you missed, as this drastically increases the risk of severe side effects.
Potential Effects of Skipping a Dose
Missing a single weekly dose of Methotrexate does not typically cause immediate harm, especially if you have been on the medication for an extended period. However, repeated or prolonged interruption of the treatment regimen can lead to a loss of therapeutic effect and disease control. Methotrexate works to suppress an overactive immune system, and when drug levels drop below the therapeutic threshold, the underlying disease can reactivate.
For individuals with autoimmune conditions, this loss of control may result in a disease flare-up, which can manifest as increased joint pain, swelling, stiffness, or worsening skin symptoms in psoriasis. Studies suggest that patients who discontinue MTX for several weeks have a significantly higher rate of disease flare compared to those who maintain continuous treatment. Even a single missed dose can delay the time it takes to reach the maximum beneficial effect, which typically takes several weeks or months to achieve.
Furthermore, attempting to correct a missed dose incorrectly poses a serious toxicity risk. If a patient takes the dose too late, or accidentally takes a double dose, the drug concentration can spike, leading to bone marrow suppression. This condition reduces the body’s ability to produce healthy blood cells, which can result in severe infection, unexplained bruising, or bleeding.
When to Contact Your Healthcare Provider
Contacting your healthcare provider or pharmacist is the appropriate action if you are unsure about the time elapsed or your specific dosing schedule. You should immediately call your provider if you suspect you have accidentally taken a double dose or mistakenly took the medication daily instead of weekly. These scenarios require urgent medical guidance to manage potential overdose and toxicity.
Specific symptoms of severe toxicity necessitate an emergency call to your healthcare team, as they may indicate a life-threatening reaction. These signs include:
- Developing severe mouth sores or ulcers.
- Unexplained bleeding or bruising.
- Persistent fever, or a severe, non-healing sore throat.
- A persistent dry cough or shortness of breath, which may signal a rare but serious lung complication.
If your underlying condition flares severely after skipping the dose, your provider may need to adjust your treatment plan or provide short-term relief medication.