Understanding Cephalexin and Its Purpose
Cephalexin, often known by the brand name Keflex, is a first-generation cephalosporin antibiotic used to treat a wide array of bacterial infections [1.10.3, 1.8.4]. It is effective against various gram-positive and some gram-negative bacteria [1.2.5]. Cephalexin works by inhibiting the formation of the bacterial cell wall, which is essential for the bacteria's survival. This action ultimately leads to the death of the bacteria, clearing up the infection [1.7.1, 1.8.2].
This medication is prescribed for numerous conditions, including [1.10.1, 1.10.2, 1.10.4]:
- Respiratory tract infections like pneumonia and pharyngitis (sore throat) caused by Streptococcus pneumoniae or Streptococcus pyogenes.
- Urinary tract infections (UTIs), including acute prostatitis, caused by bacteria such as E. coli and Klebsiella pneumoniae.
- Skin and soft tissue infections, such as cellulitis, caused by Staphylococcus aureus or Streptococcus pyogenes.
- Bone infections.
- Otitis media (middle ear infections).
Cephalexin is a broad-spectrum antibiotic, but it is not effective against viral infections like the common cold or flu [1.10.3]. A healthcare provider may take a culture to ensure the infection is caused by bacteria susceptible to cephalexin [1.3.2].
How Quickly Does Cephalexin Start Working?
Cephalexin is absorbed rapidly and reaches its peak concentration in the blood within about one hour of being taken [1.3.3, 1.7.1]. While the medication begins fighting the bacteria almost immediately, it typically takes one to three days for patients to notice a tangible improvement in their symptoms [1.2.2, 1.2.4]. The exact timeline can vary depending on the type and severity of the infection.
- For UTIs: Many patients start to feel relief from symptoms like painful urination and abdominal pain within one to two days [1.4.1].
- For Skin Infections (e.g., Cellulitis): Noticeable improvement, such as reduced redness, swelling, and pain, may take two to three days [1.2.1, 1.3.5].
- For Other Infections: Most people will start to feel better within a few days of starting the medication [1.2.4].
Primary Indicators Your Cephalexin is Effective
The most reliable sign that cephalexin is working is the reduction and resolution of your initial symptoms. Depending on your specific infection, these positive indicators can include:
- Fever Reduction: If the infection caused a fever, a noticeable decrease in body temperature is a strong sign the antibiotic is taking effect.
- Pain Relief: A decrease in localized pain, such as the burning sensation during a UTI or the tenderness of a skin infection, indicates the inflammation is subsiding [1.4.1, 1.3.5].
- Visible Improvement in Skin Infections: For conditions like cellulitis, you should see a reduction in redness, swelling, and the size of the affected area [1.3.5].
- Improved Urinary Symptoms: For UTIs, a decrease in the frequent urge to urinate and less discomfort are positive signs [1.4.1].
- General Well-being: Simply starting to feel better overall, with more energy and less malaise, is a key indicator [1.2.2].
What to Do If Cephalexin Isn't Working
If you do not start to feel better after taking cephalexin for two to three days, or if your symptoms worsen at any point, it is crucial to contact your doctor [1.2.4, 1.6.1]. Do not simply stop taking the medication. Worsening symptoms could include a persistent or higher fever, increased pain, or the spread of a skin infection.
There are several reasons why an antibiotic might not work:
- Antibiotic Resistance: The bacteria causing the infection may be resistant to cephalexin [1.7.2].
- Incorrect Diagnosis: The infection may be viral or fungal, not bacterial, in which case an antibiotic would be ineffective [1.10.1].
- Incorrect Dosage: The prescribed dose might not be sufficient to fight the infection.
If cephalexin is not effective, your healthcare provider may prescribe a different antibiotic, such as Augmentin, Bactrim, or Cipro, depending on the specific infection and potential bacterial resistance [1.6.3].
Comparison of Common Antibiotics
| Feature | Cephalexin (Keflex) | Amoxicillin (Amoxil) |
|---|---|---|
| Antibiotic Class | First-Generation Cephalosporin | Penicillin [1.8.1] |
| Mechanism | Inhibits bacterial cell wall synthesis | Inhibits bacterial cell wall synthesis [1.8.2] |
| Common Uses | UTIs, skin/bone infections, respiratory infections [1.10.2] | Ear/throat infections, pneumonia, UTIs, skin infections [1.8.3] |
| Dosing Frequency | Typically 2 to 4 times a day [1.8.1] | Typically 2 to 3 times a day [1.8.1] |
| Common Side Effects | Diarrhea, nausea, vomiting, stomach pain [1.5.1, 1.8.4] | Diarrhea, nausea, vomiting, rash [1.8.1] |
| Penicillin Allergy | Lower risk of cross-reactivity (about 10%) [1.3.3, 1.8.3] | Should not be taken by those with a penicillin allergy [1.8.1] |
The Critical Importance of Completing the Full Course
Even if you start to feel significantly better after a few days, it is essential to take the full course of cephalexin as prescribed by your doctor [1.2.4, 1.6.1]. Treatment courses typically last for 7 to 14 days [1.4.1].
Stopping your antibiotics early can lead to several negative consequences:
- Infection Relapse: Not all bacteria may be killed, allowing the infection to return, sometimes in a more severe form [1.2.4].
- Antibiotic Resistance: The surviving bacteria can be exposed to a non-lethal dose of the antibiotic, giving them a chance to develop resistance. These resistant bacteria can then multiply and cause future infections that are much harder to treat [1.11.2, 1.11.3].
Antibiotic resistance is a serious global health issue, and completing your prescribed course is a crucial step in combating its spread [1.11.2].
Conclusion
Knowing if cephalexin is working primarily involves monitoring for the improvement of your specific infection symptoms within one to three days of starting treatment [1.2.2]. Key signs include reduced fever, pain, and swelling. If you see no improvement or your condition worsens, consult your healthcare provider promptly [1.6.1]. Always complete the full prescribed course, even after you feel better, to ensure the infection is completely eradicated and to help prevent the development of antibiotic resistance [1.11.1].
For more information on Cephalexin, you can visit the National Library of Medicine's page on the drug.
