Doxycycline treats a variety of human pests, but it will not work forever

Doxycycline is an antibiotic that kills a wide, crazy and wonderful array of insects that are often difficult to treat with other antibiotics. These include bacteria and parasites that colonize our cells (called "intracellular organisms") and are difficult to access for most antibiotics.

Unlike many other antibiotics, doxycycline penetrates deep into our tissues and enters our cells where it can kill these insects. Examples of intracellular organisms susceptible to doxycycline include numerous "zoonotic infections" (animal-to-human infections), chlamydia, legionella (the cause of Legionnaires' disease) and malaria.

Other susceptible microorganisms are "spirochaetes" (which can cause syphilis and Lyme disease) and bacteria that cause acne, anthrax and cholera.
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Mechanism

Doxycycline impairs the ability of a microorganism to produce proteins - the "building blocks" of life. Protein production takes place in a part of the cell called the "ribosome," and is fundamental to the survival of any organism.
The reason why Doxycycline kills bacteria and parasites, but not our own cells, is that our ribosomes have a different type than these simpler organisms.

Used

Since doxycycline kills a variety of bacteria that can infect the airways, it is often prescribed for pneumonia and bronchitis. It is also widely used to treat acne and urinary and genital infections.
It is usually taken orally as tablets or capsules but may occasionally be given as an intravenous injection.

Doxycycline affects the ability of a microorganism to produce proteins - which is vital. Tim Proctor / Flickr, CC BY
Doxycycline works for some time after ingestion. This means that it can be used not only for treatment, but also for prevention or "prophylaxis".
Its most widespread use as prophylaxis is for tourists and other travelers (such as military personnel) who travel to tropical countries, where it is used primarily for protection against malaria. It can also provide additional protection against common bacterial causes of diarrhea.

To effectively prevent infection, it must be taken once a day while the person is at risk. Doxycycline is also effective against a number of bacteria that could possibly be used as agents of "germ" warfare. This mainly involved anthrax. Thus, it could be used as prophylaxis in military or other populations at risk of bio-war or terrorist release of anthrax into the environment.

Development

The development of doxycycline followed the momentous discovery of penicillin, a natural compound produced by a certain type of mould.
This lead many pharmaceutical companies to investigate the microbe-killing properties of a large number of other natural products, such as those produced by other microorganisms and plants, a process termed “bio-discovery”.
This unearthed natural compounds with anti-microbial activity and further synthetic modification improved these natural compounds.

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Resistance

Like all antibiotics, doxycycline is susceptible to bugs that develop resistance. There is evidence this has already occurred in settings where the drug is widely used, such as treatment of acne.
This means its use may be curtailed or overtaken by alternative drugs for some conditions, now or in the future.

Side effects and reactions

The most commonly reported side effect is inflammation of the oesophagus (food pipe), causing heartburn. This can be quite unpleasant but is somewhat preventable by taking the medication with plenty of water, while standing and well before going to bed.
“Photosensitivity” (heightened sensitivity to sunlight resulting in being easily sunburnt) is also common ( in up to 20% of people taking it). This is especially problematic for travellers using it as malaria prophylaxis in tropical countries.

Doxycycline should not be used in children or in pregnant women where it can result in permanent brown staining of teeth and have other effects on foetal bone development.
Doxycycline can increase the toxicity of the anti-inflammatory drug methotrexate.

Controversies

Recent high-profile controversies regarding side-effects from antimalarial drug mefloquine in defence-force personnel and refugees have highlighted the role of doxycycline as one of two main alternatives to mefloquine.
It is now generally considered a preferable initial choice to mefloquine for malaria prophylaxis. Interestingly, previous studies suggest many people actually prefer taking mefloquine to doxycycline.
This may reflect the nature of doxycycline’s side effects, but also its less convenient daily dosing (mefloquine is taken weekly).

Possible future uses

It has recently been found doxycycline affects processes in human cells, especially a group of enzymes important for the body’s inflammatory response. This property may be beneficial and could lead to applications for treating various non-infectious conditions.
These include cancers (especially those involving bone), inflammatory and autoimmune conditions (including rheumatoid arthritis and multiple sclerosis) and atherosclerotic diseases (plaque build-up in your arteries that can cause heart disease). However, these applications are currently still mostly in the experimental stage.