Medical uses:
Doxycycline Hyclate is a broad-spectrum tetracycline antibiotic. This drug is used against a wide variety of bacterial infections, including Rocky Mountain spotted fever and other fevers caused by ticks, fleas, and lice; urinary tract infections; trachoma; and some gonococcal infections in adults. It is an approved treatment for inhalational anthrax. It is also used with other medications to treat severe acne and amoebic dysentery (diarrhea caused by severe parasitic infection of the intestines).

Doxycycline may also be taken for the prevention of malaria on foreign trips of less than 4 months' duration. Occasionally doctors prescribe doxycycline to treat early Lyme disease and to prevent "traveler's diarrhea." These are not yet officially approved uses for this medication. It works by preventing the growth and spread of bacteria.

Doxycycline dosage:

While administration with food may decrease GI absorption of doxycycline by up to 20%, administration on an empty stomach is not recommended due to GI intolerance.

Children >8 years (<45 kg): Oral, I.V.: 2-5 mg/kg/day in 1-2 divided doses, not to exceed 200 mg/day

Children >8 years (>45 kg) and Adults: Oral, I.V.: 100-200 mg/day in 1-2 divided doses

Brucellosis: Oral: 100 mg twice daily for 6 weeks with rifampin or streptomycin
Chlamydial infections, uncomplicated: Oral: 100 mg twice daily for 7 days (or more)
Community-acquired pneumonia, bronchitis: Oral, I.V.: 100 mg twice daily
Endometritis, salpingitis, parametritis, or peritonitis: I.V.: 100 mg twice daily with cefoxitin 2 g every 6 hours for 4 days and for 48 hours after patient improves; then continue with oral therapy 100 mg twice daily to complete a 10- to 14-day course of therapy
Gonococcal infection, acute (PID) in combination with another antibiotic: I.V.: 100 mg every 12 hours until improved, followed by 100 mg orally twice daily to complete 14 days
Lyme disease, Q fever, or Tularemia: Oral: 100 mg twice daily for 14-21 days
Malaria prophylaxis: PO 100 mg daily, beginning 1 to 2 days before travel and continuing for 4 wk after leaving area.
Periodontitis: 20 mg twice daily as an adjunct following scaling and root planing; may be administered for up to 9 months. Safety beyond 12 months of treatment and efficacy beyond 9 months of treatment have not been established.
Rickettsial disease or ehrlichiosis: Oral, I.V.: 100 mg twice daily for 7-14 days
Rosacea: Oral: 40 mg once daily in the morning
Sclerosing agent for pleural effusion injection (unlabeled use): Irrigation: 500 mg as a single dose in 30-50 mL of NS or SWI
Syphilis: Early syphilis: Oral, I.V.: 200 mg/day in divided doses for 14 days Late syphilis: Oral, I.V.: 200 mg/day in divided doses for 28 days
Yersinia pestis (plague): Oral: 100 mg twice daily for 10 days
Vibrio cholerae: Oral: 300 mg as a single dose
Inhalation Anthrax (Post-exposure):
Adults and children (100 lb [45 kg] or more): PO 100 mg bid for 60 days.
Children (less than 100 lb [45 kg]): PO 2.2 mg/kg bid for 60 days.

Doxycycline side effects:

Gastrointestinal: anorexia, nausea, vomiting, diarrhea, glossitis, dysphagia, enterocolitis, and inflammatory lesions (with monilial overgrowth) in the anogenital region. Hepatotoxicity has been reported rarely. Rare instances of esophagitis and esophageal ulcerations have been reported in patients receiving capsule and tablet forms of the drugs in the tetracycline class. Most of these patients took medications immediately before going to bed.
Skin: maculopapular and erythematous rashes; exfoliative dermatitis; photosensitivity.
Renal toxicity: Rise in BUN has been reported and is apparently dose related.
Hypersensitivity reactions: urticaria, angioneurotic edema, anaphylaxis, anaphylactoid purpura, serum sickness, pericarditis, and exacerbation of systemic lupus erythematosus.
Blood: Hemolytic anemia, thrombocytopenia, neutropenia, and eosinophilia have been reported.
CNS: dizziness; headache; pseudotumor cerebri (manifested by headache and blurred vision).
Other: bulging fontanels in infants and intracranial hypertension in adults.

Precautions:

Don't take if: you have ever had an allergic to any tetracycline antibiotic.
Before taking Doxycycline, tell your doctor or/and pharmacist:

if you have kidney or liver disease, patients with severe kidney disease may have an increased chance of side effects
if you have lupus
if you have myasthenia gravis
if you have diabete

Skin & sunlight: May cause rash or intensify sunburn in areas exposed to sun or ultraviolet light. Avoid overexposure. Notify doctor if reaction occurs.
Driving, piloting or hazardous work: No problems expected
Pregnancy: Risk category D. Risk to unborn child outweighsdrug benefits. Don't use.
Breast feeding: Drug passes into milk. Avoid drug or discontinue nursing until you finish medication. Consult doctor for advice on maintaining milk supply.
Discontinuing: Don't discontinue prescrition without consulting doctor until you complete prescribed dose, even though symptoms diminish or disappear.
Do not take doxycycline just before going to bed. It may not dissolve properly when you are lying down and can cause pain in your throat.
Birth control pills may not be effective. Use additional birth control method.

Drug interactions

Antacids (containing aluminum, calcium, or magnesium) - decreased Doxycycline effect
Anticoagulants, oral - increased anticoagulant effect
Antivirals (decreased antibiotic effect)
Barbiturates - may decrease the level/effect of doxycycline.
Bile acid sequestrants, Bismuth subsalicylate - decreased Doxycycline absorption
Calcium supplements - decreased antibiotic effect
Carbamazepine - may decrease the level/effect of doxycycline
Cefixime - decreased effect of cefixime
Cholestyramine or colestipol - decreased antibiotic effect
Coumarin derivatives - doxycycline may enhance the anticoagulant effect of coumarin derivatives.
Desmopressin - possible decreased desmopressin effect
Digitalis preparations - increased digitalis effect
Etretinate - increased chance of adverse reactions of etretinate
Iron-containing products - decreased absorption of drug
Lithium - increased lithium effect
Mineral supplements (calcium, magnesiun, zinc) - decreased doxycycline absorption, separate doses by 1 to 2 hours
Methotrexate - increased serum concentration of methotrexate.
Methoxyflurane - concomitant use may cause fatal renal toxicity. Avoid concurrent use.
Oral contraceptives - there is little evidence that tetracyclines reduce the efficacy of the oral contraceptive pill unless they cause gastrointestinal upset.
Penicillins - decreased penicillin effect
Pimecrolimus - Doxycycline may decrease the metabolism, via CYP isoenzymes, of pimecrolimus.
Quinapril - decreased the absorption of antibiotic
Retinoic acid derivatives - Doxycycline may enhance the adverse/toxic effect of retinoic acid derivatives. The development of pseudotumor cerebri is of particular concern.
Typhoid vaccine - antibiotics may diminish the therapeutic effect of typhoid vaccine. Only the live attenuated Ty21a strain is affected.
Sodium bicarbonate - greatly reduced doxycycline absorption
Vitamin A - increased risk of intracranial hypertension
CYP3A4 inducers (aminoglutethimide, carbamazepine, nafcillin, nevirapine, phenobarbital, phenytoin, and rifamycins) - may decrease the levels/effects of doxycycline.
CYP3A4 substrates (benzodiazepines, calcium channel blockers, mirtazapine, nateglinide, nefazodone, tacrolimus, and venlafaxine) - Doxycycline may increase the levels/effects of CYP3A4 substrates. Selected benzodiazepines (midazolam and triazolam), cisapride, ergot alkaloids, selected HMG-CoA reductase inhibitors (lovastatin and simvastatin), and pimozide are generally contraindicated with strong CYP3A4 inhibitors.

Possible interactions with other substances:

Alcohol - possible liver damage, avoid.
Food - serum levels may be slightly decreased if medication taken with food or milk. Administration with iron or calcium may decrease doxycycline absorption. May decrease absorption of calcium, iron, magnesium, zinc, and amino acids.
St John's wort may decrease doxycycline levels. Avoid dong quai, St John's wort (may also cause photosensitization).

Test Interactions

False elevations of urine catecholamine levels
False-negative urine glucose using Clinistix®, Tes-Tape®

Pregnancy & Lactation:
Pregnancy Risk Factor: D
Exposure during the last half or pregnancy causes permanent yellow-gray-brown discoloration of the teeth. Tetracyclines also form a complex in bone-forming tissue, leading to a decreased fibula growth rate when given to premature infants. According to the FDA, the Teratogen Information System concluded that therapeutic doses during pregnancy are unlikely to produce substantial teratogenic risk, but data are insufficient to say that there is no risk. In general, reports of exposure have been limited to short durations of therapy in the first trimester.

Breast-Feeding: Doxycycline is excreted in breast milk. Breast-feeding is not recommended by the manufacturer.
The drug is less bound to the calcium in maternal milk which may lead to increased absorption compared to other tetracyclines. Only minimal amounts of doxycycline are excreted in human milk and the relative amount of tooth staining has been reported to be lower when compared to other tetracycline analogs. Nondose-related effects could include modification of bowel flora.
For short term use, doxycycline is generally considered compatible (low risk to fetus) during breast-feeding.
For chronic use, doxycycline should be used with caution (high risk to fetus) during breast-feeding.

Overdose:
If overdose is suspected, contact your local poison control center or emergency room immediately. Symptoms: severe nausea, vomiting, diarrhea.

Important to know:

If you have no medical prescription you can still purchase antibiotic online. However, if your symptoms don't improve after the treatment, you need to consult your physician to change the prescription.

References:

1. U.S. Food and Drug Administration. Doxycycline hyclate (Vibramycin) U.S. Prescribing Information. Available at (PDF format): Prescribing Information