Along with its needed effects, doxycycline may cause some unwanted or undesirable effects. Generally, doxycycline 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
• vomiting
• nausea
• gas
• stomach upset
• appetite loss
• discoloration of bones and teeth in children under 8 years of age and in children of mothers given doxycycline during pregnancy

Uncommon:

• sensitivity to sunlight
• allergic reaction (e.g. rash, itching, difficulty breathing)
• inflammation of the lining of the mouth
• inflammation of the tongue

Rare:

• esophageal (food pipe) ulceration
• sore mouth and tongue

Unwanted effects associated with the gut should be reported to your pharmacist if they persist or become bothersome.

Doxycycline is an antibiotic medication. It works by inhibiting the bacteria's production of protein. This slows and prevents it's growth and reproduction. The effectiveness of doxycycline depends on factors such as dose, concentration in the blood as well as other body fluids and tissue, and susceptibility of the organism. Infections caused by bacteria that are not sensitive to doxycycline will not show improvement after taking this medication.

Allergic reactions can occur with doxycycline 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.
Use of doxycycline in children less than 8 years of age is not recommended because safe conditions for its use have not been established. Use of doxycycline in this age group, during tooth development, has been known to cause permanent tooth discoloration. It has also been known to decrease bone growth when given to premature infants.
Avoid prolonged exposure to direct sunlight or tanning beds. Some people may develop a sensitivity to sunlight while taking doxycycline that usually presents itself in the form of a rash and/or painful burn. Sunscreens and protective clothing should be worn as a precautionary measure when exposed to direct sunlight.

Doxycycline belongs to the tetracycline group of antibiotics. If you are allergic to a tetracycline (e.g. minocycline, tetracycline) you may also be allergic to doxycycline.
Antacids containing aluminum, magnesium or calcium and oral iron preparations interfere with the absorption of doxycycline and should be avoided.
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 doxycycline may need to be stopped and another antibiotic prescribed to treat the new infection.
Diarrhea sometimes develops while taking doxycycline. 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 doxycycline:

• antacids containing aluminum, magnesium or calcium
• bismuth salts (e.g. Pepto-Bismol®)
• warfarin (e.g. Coumadin®)
• penicillin
• carbamazepine (e.g. Tegretol®)
• digoxin (e.g. Lanoxin®)
• oral iron preparations (e.g. Palafer®)

Use is not recommended in the following situations:

• allergy to any tetracycline (e.g. minocycline)

Caution is recommended in the following situations:

• children under 8 years of age

Use in pregnancy: Doxycycline is not recommended during pregnancy because of possible adverse effects on developing bones and teeth of the unborn baby. Consult your doctor or pharmacist if you suspect you are pregnant.
Use while breastfeeding: Doxycycline does appear in breast-milk. Consult your doctor or pharmacist before use.