What are the main risks or side effects, and how often do they happen?

The main risk is stomach upset, and at the low 20 mg dose it is uncommon and usually mild.

The best safety data at this exact dose comes from a two-year placebo-controlled trial in 128 postmenopausal women taking subantimicrobial-dose doxycycline (20 mg twice daily): no significant adverse events were attributed to the drug, and skin-related side effects were actually less frequent than on placebo [4]. A six-month placebo-controlled trial in 20 diabetic periodontitis patients reported no adverse reactions and no dropouts [4]. Those are small trials, and neither was built to catch rare harms.

The stomach numbers come from higher, standard doses, not your dose. In 141 adolescents on full-dose doxycycline for pelvic inflammatory disease, 86 had no stomach problems, 45 needed more food with the pill, and 10 had significant side effects [1]. In a healthy-volunteer comparison, nausea hit 59 people on plain doxycycline versus 21 on an enteric-coated version and 10 on placebo [2]. Enteric coating and taking it with food cut that sharply [2].

One lab finding deserves a flag. In isolated mouse and human nerve cells, doxycycline triggered a calcium signal through mitochondrial reactive oxygen species, a mechanism that could underlie nerve-related side effects [3]. That is a dish experiment at concentrations above what 20 mg produces, not proof of harm in people.

My call: at this dose the known risks are mild stomach upset and, rarely, skin sensitivity, and the two-year data is the strongest safety signal here, though it is thin on rare events. Confidence: moderate.