I was prescribed cephalexin for staph but it gave me an allergic reactions. i told my doctor i have been taking it for 4 days but its was 3 day. he prescribed me doxycycline will it still be affective he prescribed me doxycycline 100mg for 5 days?

At some point you have to trust your medical practicioner to be doing a good job for you. PLease ask them this question, that is what you pay them for! When sensitivity reactions occur to antibiotics it is customary to prescribe an alternative compound right away to prevent sensitization of the causative bacteria to the first medication. https://academic.oup.com/cid/article/58/8/1140/355853

Doxycycline is active against most strains of Staphylococcus and usually is fine for relatively non-serious staph infections, such as those that do not require hospitalization and can be treated with oral antibiotics. You can trust your doctor's advice and decision -- but if in doubt, contact the office to discuss it.