Doxycycline

Key facts

• Doxycycline is a tetracycline antibiotic that stops susceptible bacteria making the proteins they need to multiply.
• It's taken once or twice a day, sitting or standing upright with a full glass of water, and you should stay upright for at least half an hour afterward.
• Avoid it in pregnancy and in young children: it can permanently discolour developing teeth and affect bone growth.
• It causes marked sun sensitivity, use strong sunscreen and cover up, since sunburn can happen quickly, even on a cloudy day.

What Doxycycline treats

Doxycycline treats acne, chest infections, sinusitis, urinary tract infections, chlamydia and other sexually transmitted infections, early Lyme disease, malaria prevention, and travelers' diarrhoea. It's bacteriostatic, meaning it stops bacteria multiplying rather than killing them outright, so recovery also depends on your own immune system.

How Doxycycline works

Doxycycline enters bacterial cells and attaches to their ribosomes, the machinery that builds proteins. This blocks bacteria from making the proteins they need to grow and divide, holding the infection in check while your immune system clears the rest.

Before you take it

• Do not take doxycycline if you're pregnant, or give it to a young child, except where a doctor judges it necessary; it binds to calcium in developing teeth and bone and causes permanent staining.
• Take it with a full glass of water while sitting or standing; lying down straight after a dose can let the capsule lodge in your oesophagus and cause ulceration.
• Antacids, iron and calcium supplements, and dairy products block absorption; take doxycycline at least two hours apart from these.
• Tell your prescriber about liver disease or a tetracycline allergy.

Side effects

Nausea, diarrhoea, loss of appetite and a mild rash are common.

Stop and seek urgent medical care for any of these:

• Severe sunburn or blistering after brief sun exposure.
• Difficulty or pain swallowing, or chest pain.
• Yellowing of the skin or eyes, or a severe headache with visual disturbance.
• Hives, facial swelling or difficulty breathing.

Safety essentials

• Avoid direct sun and use high-SPF sunscreen throughout treatment; doxycycline-related sunburn can happen fast and be severe.
• Never take doxycycline during pregnancy or give it to a young child, it permanently discolours developing teeth and can affect bone growth; safer antibiotics are used instead.
• Take each dose upright with a full glass of water, and stay upright for 30 minutes afterward, to protect your oesophagus.
• Finish the full course even once you feel better; stopping early can let the infection return and adds to antibiotic resistance.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.