Doxycycline

Doxycycline belongs to the tetracycline family. It blocks the production of proteins that bacteria need to live and multiply. To do this, it binds to the 30S subunit of the ribosome, the machine that translates the genetic message into proteins. By occupying this site, it prevents the charged aminoacyl-tRNA from taking its place on the ribosome’s A site, which interrupts the elongation phase of translation. Its action is bacteriostatic: it halts the growth of bacteria rather than destroying them directly, and lets the body’s defences finish the job.

Some bacteria escape doxycycline thanks to ribosomal protection proteins, notably Tet(O) and Tet(M). These proteins dislodge the antibiotic from its binding site, and the ribosome then resumes its activity. This is one of the reasons an antibiotic is prescribed only for a documented bacterial infection, and why the prescribed duration must be followed through to the end.

Doxycycline treats specific bacterial infections. It is prescribed in particular against Chlamydia infections, Rocky Mountain spotted fever and other tick-borne rickettsial diseases, as well as respiratory infections caused by Mycoplasma pneumoniae. It is also used as adjunctive treatment for severe acne and to prevent malaria during travel to at-risk areas. The choice of the molecule depends on the target organism and the clinical context.

Its long half-life, from 18 to 22 hours, generally allows one to two doses per day. After an oral dose, it passes almost entirely into the blood, with a bioavailability of about 95 to 100 percent, and reaches its peak concentration at around two hours. About 40 percent is eliminated by the kidney, with minimal hepatic metabolism, which makes it usable even when kidney function is reduced.

Swallow each dose with a large glass of water. Rare cases of inflammation and ulceration of the oesophagus have been reported with capsules and tablets, when the medicine lingers in the oesophagus. Stay upright, sitting or standing, during and after taking it. If the medicine causes stomach discomfort, you can swallow it with a little food or milk: for doxycycline, the effect of milk on absorption remains moderate. Even so, keep large calcium intakes away from the time of the dose.

Take doxycycline at regular times and complete the full prescribed duration, even if you feel better before the end: stopping too soon encourages relapse and resistance. During treatment, your skin becomes more sensitive to the sun and may react like an exaggerated sunburn; protect yourself, and at the first sign of skin redness, stop the treatment and seek advice. Doxycycline also permanently stains developing teeth, which is why it is usually avoided before the age of 8 and during pregnancy.

The most frequent effects, between 1 case in 100 and 1 in 10, are nausea, vomiting and vaginal candidiasis. Less often, between 1 in 1,000 and 1 in 100, there is difficult digestion, headaches, tinnitus, photosensitivity reactions and skin eruptions, as well as blood abnormalities: haemolytic anaemia, a drop in white blood cells or platelets.

More rarely, between 1 in 10,000 and 1 in 1,000, pancreatitis, Clostridium difficile colitis, an ulcer or inflammation of the oesophagus, benign intracranial hypertension, liver damage and severe skin reactions such as Stevens-Johnson syndrome, toxic epidermal necrolysis and erythema multiforme have been described. These serious effects remain exceptional, but they justify seeking advice without delay in the face of unusual pain, a widespread eruption or persistent headaches with visual disturbances.

Several minerals bind to doxycycline in the digestive tract and prevent it from being absorbed: calcium, magnesium, iron, zinc and aluminium form complexes with it. Separate antacids and calcium or magnesium supplements by at least 1 to 2 hours, and space iron out by 3 hours before or 2 hours after the dose. Tetracyclines also reduce prothrombin activity: in a person on warfarin, the prothrombin time lengthens and the anticoagulant dose sometimes has to be reduced. Doxycycline can also make oral contraceptives less effective, and enzyme inducers such as barbiturates, carbamazepine and phenytoin shorten its half-life.

With prolonged treatment, periodic laboratory tests monitor the blood, the kidney and the liver. Tell your doctor about all the medicines and supplements you take, because these interactions often come down to the timing of doses. Do not change or stop doxycycline on your own: only the prescriber adjusts the dose or the duration.