Tavegyl is the same thing as Tavist, which is an antihistamine. See http://www.drugs.com/tavist.html
Histamine is a neurotransmitter as well as a chemical messenger involved with the immune system. Suppression of histamine therefore can have effects on the immune system and nervous system.
There are some contra-indications and medication interactions with Tavist that may have triggered your symptoms. I would discuss this with a pharmacist if I were you - they are usually more savvy about drug side-effects and interactions than most MD's.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Pancytopenia, chronic malabsorption/malnutrition, etc.; G-Tube; Currently TPN-dependent.
Meds: Zofran, Pulmicort, IV Ceftazidime, Heparin (to flush PICC line), Claritin, Colloidal Silver (used topically), probiotics, homeopathy.