Minocycline and doxycycline are both broad spectrum, long-acting, semi-synthetic antibiotics in the tetracycline family. Both are bacteriostatic, in that they prevent bacteria from reproducing by interfering with protein synthesis. They are used to treat similar conditions, including bacterial and protozoal infections. There are many differences, however, between these two drugs. They have some different specialties, spectrums of action, side effects and safety issues.
Doxycycline is also used as an antihelmintic or anti-worm medication in addition to its uses for bacterial and protozoal infections. It is a more bioactive medication than the other tetracycline antibiotics, including minocycline. Conversely, minocycline is a broader spectrum drug than doxycycline and is used against a wider variety of bacteria. Minocycline is also used to treat rheumatoid arthritis and is recommended in this role by the American College of Rheumatology.
Other differences between minocycline and doxycycline include their solubility and other chemical characteristics. Minocycline is more soluble in fats while doxycycline more easily dissolves in water. For this reason, minocycline crosses the blood-brain barrier more easily than doxycycline and causes more central nervous system (CNS) side effects. Doxycycline, as it is more soluble in water, is the better choice for urinary tract infections as its active metabolites are dissolved in and excreted via urine. This drug should not be taken with calcium supplements or calcium-rich foods that block absorption, while there is no such restriction with minocycline.
Some side effects differ between these medications as well. Minocycline's greater solubility in lipids causes more common central nervous system side effects, such as dizziness and lethargy. A known but rare side effect of this drug is secondary intracranial hypertension, a serious condition first indicated by headaches, disorientation and dizziness. It has also been linked to the development or exacerbation of systemic lupus and thyroid cancer. The relationship between these two conditions and minocycline has not been established as causative, but experts caution careful evaluation while on this medication.
As with all tetracycline antibiotics, minocycline and doxycycline can both cause increased photosensitivity — or sunburn — with nominal sun exposure. Unlike the other tetracycline antibiotics that can sometimes cause significant kidney damage, however, either can usually be safely used in patients with impaired renal function. All drugs in the the tetracycline family of antibiotics should never be taken or administered after the medications' expiration dates. Ingredients used in the manufacture of these medicines break down over time to form toxins that are particularly dangerous to the kidneys.