Along with its needed effects, amoxicillin may cause some unwanted or undesirable effects. Generally, amoxicillin 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

Amoxicillin is an antibiotic medication that kills various bacteria. It works by inhibiting the synthesis of one of the building blocks needed for the bacteria to make its cell wall. This results in a cell wall that is defective and more likely to rupture. The effectiveness of amoxicillin depends on factors such as dose, concentration in the blood as well as other body fluids and tissue, and susceptibility of the organism. Amoxicillin will only kill certain types of bacteria sensitive to its antibiotic action. Infections caused by bacteria that are not sensitive to amoxicillin will not show improvement after taking this medication.

Allergic reactions can occur with amoxicillin 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.
Amoxicillin is a semisynthetic penicillin. If you have an allergy to penicillin you should not take amoxicillin. 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, the amoxicillin may need to be stopped and another medication prescribed to treat the new infection.
Diarrhea often develops while taking amoxicillin. 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.
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.
The following drugs or drug classes have been known to interact with amoxicillin:

• probenecid 
• erythromycin
• tetracycline
• methotrexate
• allopurinol
• warfarin 

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

Use in pregnancy: Amoxicillin has been used in pregnant women without evidence of risk to the unborn baby. 
Use while breastfeeding: Amoxicillin does appear in low concentrations in breastmilk. It is generally considered safe to use while breast-feeding.