Along with its needed effects, amoxicillin / clavulanate may cause some unwanted or undesirable effects. Generally, it is well tolerated and many people will not experience unwanted effects. The frequency and severity of these effects is dependant on many factors including dose, duration of therapy and individual susceptibility.

Possible unwanted effects include:

• diarrhea
• allergy (e.g. hives, itching, rash, difficulty breathing)
• nausea
• headache

Uncommon: 

• stomach upset
• vomiting
• appetite loss
• inflammation of the tongue

Rare:

• bloody diarrhea
• pseudomembranous colitis (inflammation of the colon caused by the overgrowth and toxic production of certain bacteria, causing diarrhea)
• sore mouth and tongue
• black hairy tongue

Clavulanic acid is a medication that is sometimes added to penicillins to prevent bacteria from becoming resistant to the penicillin.  It is a beta-lactamase inhibitor.  Although not effective as an antibiotic on its own, it does increase the effectiveness of antibiotics such as amoxicillin as in a product known as Clavulin®.

Allergic reactions can occur with amoxicillin / clavulanate use. People with a history of allergy, asthma, hay fever or hives seem to be more susceptible to these reactions. The reaction can be immediate and severe. Allergic symptoms include wheezing, hives, itching, swelling, spasms in the throat and breathing tubes, joint and muscle pain, difficulty breathing, fever and skin rashes. Nausea and vomiting are not symptoms of an allergic reaction.
If you have an allergy to penicillin you should not take amoxicillin / clavulanate. Cephalosporins (e.g. cephalexin, cefaclor) are a distinct group of antibiotics related to penicillins. People allergic to cephalosporins may also be allergic to penicilins.
Taking the antibiotic repeatedly or for prolonged periods may result in bacterial or fungal overgrowth which can lead to a second infection. When this occurs, amoxocillin / clavulanate may need to be stopped and another medication prescribed to treat the new infection.
Diarrhea often develops while taking amoxicillin and clavulanate. This is sometimes caused by an overgrowth of bacteria in the gut that are not killed by the antibiotic. In severe cases, this may be life threatening and would require treatment with other antibiotics. In mild cases, symptoms disappear shortly after the drug is discontinued.
Clavulanic acid may cause a decrease in liver function. If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately.

Drug Interactions: It is important to tell your doctor and pharmacist of any prescription or over-the-counter medications you are taking. In some cases the dose of one or both drugs may need to be altered or another drug may be prescribed.

• probenecid 
• erythromycin
• tetracycline
• methotrexate
• allopurinol
• warfarin (e.g. Coumadin®)

Use is not recommended in the following situations:

• allergy to any penicillin.

Caution is recommended in the following situations:

• allergy to cephalosporins (e.g. cephalexin, cefaclor)
• kidney impairment
• Liver problems

Use in pregnancy: Amoxicillin / clavulanate has been used in pregnant women without evidence of risk to the unborn baby. However, it should not be used during pregnancy unless clearly needed. Consult your doctor or pharmacist if you suspect you are pregnant.
Use while breast-feeding: amoxicillin and clavulanate do appear in low concentrations in breastmilk. It is generally considered safe in breast-feeding, however consult your doctor or pharmacist before use